How to solve part (c) of this question? Thank you!

An observer watches a slowly-moving alpha-particle with speed v and mass 4 amu (atomic mass units) collide elastically with a stationary alpha-particle. After the collision, the observer sees that both particles are now moving, in different directions. (a) Draw a diagram showing the colliding particles before collision in the reference frame of the observer, and in the zero momentum frame (the frame of reference where the total summed momentum of the particles is zero). (b) Draw a diagram showing the colliding particles after collision in the zero momentum frame, and in the reference frame of the observer. (c) In the zero momentum frame, what is the speed of each particle after the collision