For an elastic collision, both kinetic energy and momentum are conserved.
Conservation of momentum
m(He)*v(He,initial) = m(He)*v(He,final) + m(H)*v(H)
where m(He) is the mass of the Helium,
v(He, inital) is the initial speed of the Helium , etc. . .
Conservation of kinetic energy:
0.5*m(He)*v(He,initial)^2 = 0.5*m(He)*v(He,final)^2 + m(H)*v(H)^2
you should be able to look up the masses of a hydrogen atom and a helium atom. Then, you have two equations with 2 unknowns (v(He, final) and v(H)). You can use algebra to solve for these two values.
For the second part, repeat this problem by replacing the helium with a neutron.