Assuming your two trolleys stick together after the collision, your momentum equation would be M1V1+M2V2=(M1+M2)V3 and since V2 is 0, then 2x2=(2+2)V3, thereore the final velocity would be 1, wich would make the final momentum 4
I believe the answer for 1) is
PEgrav = mass • g • height
PEgrav = m *• g • h