A neutron with a mass of 1.67 × 10-27 kg traveling east with a kinetic energy of 2.00 × 10-21 J collides perfectly elastically with a helium nucleus with a mass 6.68 × 10-27 kg that is initially at rest. After the collision, the neutron has a kinetic energy 1.80 × 10-21 J. What angle was the neutron deflected during the collision? I am able to calculate the values of the final velocities. I just don't know what to do with the information provided and the information I calculated to get the final angle of deflection.

I understand that I can use the x and y components of momentum. However, it doesn't provide a value for the approach angle of the neutron. I tried assuming that theta of the neutron was 0 degrees so that is was simply moving along the x-axis in the positive x-direction until it hit the nucleus. However, when I then tried to solve for the theta after the collision and a resulting change in velocity the final theta came out as being undefined.