M2 help (edexcel)

A particle P of mass 0.2 kg is at rest at a fixed origin O. At time t seconds, where 0 ≤ t ≤ 3 , is a force (2ti + 3j)N applied to P.

a) Find the position vector of P when t = 3.
r = 45i +67.5j (I worked it out with no problem)

b) When t = 3, the force acting on P changes to (6i + (12 - t^2)j)N, where t ≥ 3. Find the velocity of P when t = 6.

I got as far as v = 30ti + (60t - 5/3 t^3)j+ c (by using F=ma to find acceleration then integrating it to find velocity)
I don't know how to work out C.
Help is appreciated!

From a) you can work out the velocity when t = 3.

Use that as your initial conditions for part b).