Momentum and collision problem?

Question

A neutron collides elastically with a helium nucleus (at rest initially) whose mass is four times that of the neutron. The helium nucleus is observed to rebound at an angle '2 = 45° from the neutron's initial direction. The neutron's initial speed is 6.2 105 m/s. Determine the angle at which the neutron rebounds, '1, measured from its initial direction.

Angle?

Speed of Neutron after collision-

speed of helium nucleus after collision-

Thank you

Answer 1

M He mass and m neutron mass x neutron direction
a 45 degree angle means that MVx=MVy =mvy (from conservation of momentum) also MVx+mvx=mv0
so vy+ vx=vo and from conservation of energy 1/2(vx^2+vy^2)+4vy^2=1/2v0^2 solve for vx and vy and use trig to get angle