Momentum in 2 dimensions

diagram below shows the initial state of two identical balls A and B before collision and the final state immediately after the collision. The mass of each ball is 1.2 kg.
Before the collision, the velocity of A is 3.0 m s−1 and B is stationary. A makes an oblique collision with B. After the collision, A moves off at an angle of 30° to its original direction and has a velocity of 2.6 m s−1. B is deflected at an angle θ with a velocity of 1.5 m s−1.
a Explain why the final momentum of the system in a direction at right angles to the initial velocity of A is zero. Hence determine the angle θ. [3] b Show that the total momentum of the system is conserved. [3]
I am confused in part a the first part of question. How do I get that pattern of question?
diagram below shows the initial state of two identical balls A and B before collision and the final state immediately after the collision. The mass of each ball is 1.2 kg.
Before the collision, the velocity of A is 3.0 m s−1 and B is stationary. A makes an oblique collision with B. After the collision, A moves off at an angle of 30° to its original direction and has a velocity of 2.6 m s−1. B is deflected at an angle θ with a velocity of 1.5 m s−1.
a Explain why the final momentum of the system in a direction at right angles to the initial velocity of A is zero. Hence determine the angle θ. [3] b Show that the total momentum of the system is conserved. [3]
..........I can't seem to get the first part of (a). Can someone help me?
Original post by Mariia02
diagram below shows the initial state of two identical balls A and B before collision and the final state immediately after the collision. The mass of each ball is 1.2 kg.
Before the collision, the velocity of A is 3.0 m s−1 and B is stationary. A makes an oblique collision with B. After the collision, A moves off at an angle of 30° to its original direction and has a velocity of 2.6 m s−1. B is deflected at an angle θ with a velocity of 1.5 m s−1.
a Explain why the final momentum of the system in a direction at right angles to the initial velocity of A is zero. Hence determine the angle θ. [3] b Show that the total momentum of the system is conserved. [3]
..........I can't seem to get the first part of (a). Can someone help me?

There is no diagram, so I would make some speculation on the diagram.
We shall note that momentum is a vector quantity and is conserved before and after the collision. If the initial motion of A is parallel to the x-axis, then the total momentum before the collision is along the x-axis and no momentum along the y-axis. After the collision, the momentum along the y-axis (right angles to the initial velocity) must be zero for momentum to be conserved.
To find the angle θ, resolve the component of momentum of A and B parallel to the y-axis, equate them or sum them vectorially to zero and solve for θ.

Spoiler

Original post by Mariia02
diagram below shows the initial state of two identical balls A and B before collision and the final state immediately after the collision. The mass of each ball is 1.2 kg.
Before the collision, the velocity of A is 3.0 m s−1 and B is stationary. A makes an oblique collision with B. After the collision, A moves off at an angle of 30° to its original direction and has a velocity of 2.6 m s−1. B is deflected at an angle θ with a velocity of 1.5 m s−1.
a Explain why the final momentum of the system in a direction at right angles to the initial velocity of A is zero. Hence determine the angle θ. [3] b Show that the total momentum of the system is conserved. [3]
..........I can't seem to get the first part of (a). Can someone help me?