For the orange cart,

= (1m/s + 1m/s)/2
= 1m/s. (It had better!)

Its displacement is just (1m/s) (4s) = 4m which is confirmed by our graph. Of course we don't really need to do all that since the velocity is constant.

For the blue cart,
= (0m/s + 2m/s)/2 = 1m/s.
Its displacement is also (1m/s) (4s) = 4m which is again confirmed by our graph and by our initial observation that both objects have the same displacements.