use the equation mv+mv=mv+mv
95.5(.51)= 95.2(X)+ 17.5(.290)
Now solve for X and that should be your answer
OR
you know that if you throw the rock forward then the the same amount of force is applied backwards onto the person throwing the rock so if the force applied to the rock is mass times velocity then find the force for the cart and subtract the carts force forward by the force the rock exerts backwards.