Well I am going to assume two things so tell me if I am wrong. The mass of both balls are the same and that you know the equation for momentum.
The way you start this problem and all problems like it is to break this up into components, find the sum of the momentums in the x direction and then find another equation for the sum of the momentums in the y direction.
The equation you are using is right but you will end up with two equations.
The mass is the same no matter which direction so all you are concerned with is the velocity which moves with different x and y components depending on the angle. So just use sin's and cos's to find these components.
So since the first ball is traveling horizontally initially its y component is zero. However it deflects at an angle to where the velocity has both an x and y component to it.
One more question is the 41.5% in the answer supposed to be 41.5 degrees
Well here is a general form of the equation.
[tex]M_{1}V_{1x}+M_{2}V_{2x}=M_{1}V'_{1x}+M_{2}V'_{2x}[/tex]
and another equation for the y components
[tex]M_{1}V_{1y}+M_{2}V_{2y}=M_{1}V'_{1y}+M_{2}V'_{2y}[/tex]
but in the problem they give you an angle that the first ball deflects at being 29.7 degrees below the x-axis so in order to find the x component you would simply have a Vcos(theta) to be the x component of the ball. Where V is the velocity the ball is traveling and theta is the angle. Its easiest to think about it like a triangle with the hypoteneuse going in the direction the ball is traveling and the x and y components being the opposite and adjacent sides.