Where do I start and why?

[warped skiff]

Based on the velocities before and after the collision, determine the mass ratio of the red and blue carts. Use the formula:
mass of red cart / mass of blue cart = (initial velocity of blue cart − velocity after collision) / velocity after collision.

This was just given to me without anything else. What do I even start with?

[vocal raft]

well what do you know

[wild cloak]

what happens to either cart after the crash

[unkempt cradle]

You have to use the

Conservation of momentum which applies for isolated systems. Your isolated system here is: S={red cart+blue cart}