A neutron collides elastically with a helium nucleus (at rest initially) whose mass is four times that of the?

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 move off at an angle θ'2 = 41° from the neutron's initial direction. The neutron's initial speed is 5.6 × 105 m/s. Determine the angle of the neutron, θ'1, measured from its initial direction.