Think about the angles CPB and AOB in terms of the other angles, making an equation for each angle. There is a quadrilateral OBCP, so think about internal angles in a quadrilateral. You should end up with CPB = AOB
Reasons for each step can be expressed in a few words, such as "Angles on a straight line equal 180 degrees." They do not need to be too detailed, or proven using values of angles, as you are not given any values of angles. Apart from the two right angles you have labelled, no other angle can be given an accurate value.