I am missing something. I need to determine the mass equivalence of the helium-4's kinetic energy and need to use the formula E=mc^2, solving for the...

I am missing something. I need to determine the mass equivalence of the helium-4's kinetic energy and need to use the formula E=mc^2, solving for the mass.

I also needed to determine the mass equivalence of the neutrons kinetic energy by using momentum.