Finding initial speed after a collision, answer given but don't understand explanation?

a proton collides elastically with a stationary proton. if the first proton scatters at an angle of 37 degrees from its original direction, what is the speed of the first proton after the collision? the initial speed of the first proton is 317 m/s. the answer to this question is 253 m/s but I don't get how..any help appreciated, thanks!