I think this is how you do it =D
So you need to find momentum given the wavelength of a particle...what equation is there that relates the two? (Remember that
)
For the second part, you have to remember that linear momentum is conserved in all situations. So if that photon somehow was ejected from that proton (
, the hydrogen nucleus, is just a proton - the electron isn't there anymore in the atom, giving it a net +1 charge), the proton must recoil so that momentum is conserved, like how a gun recoils when you shoot a bullet due to conservation of momentum.
So the recoil momentum and the momentum of the photon must sum up to zero to obey the law of conservation.