For the second one I would use the chain rule ie bring the square down and the 3x^2 out of the inside of the function to obtain (6x^2)(f(x^3)) I think :-)
When I do that I obviously find that v2=u-v1 and when I sub this into the conservation of energy equation it still comes out as either v1=u or v1=0, and I am assuming that this can't be the case as it states in the question that the ball continues to move after the collision at an angle θ :s
Homework Statement
A cue ball with speed U hits a stationary red ball of equal mass. The collision is elastic (ie no energy is converted into other forms). After the collision the cue ball is moving at an angle θ to its original path. Find the final speed of the cue ball.
Homework...