Projectiles. First of all you want to work out the starting velocity u, in the x and y directions. To do this you used trigonometry, to find the opposite and adjacent sides. You set the ending vertical velocity to 0ms^-1 (because you know this is 0 at maximum vertical displacement, until it later falls to the ground under gravity), and the horizontal acceleration is presumed 0 (assumption that there is no air resistance). Your acceleration vertically is due to gravity, acting against the vertical direction of the cart (-9.81ms^-2). This cart is modelled presumably as a point mass(?) with no added weight, so we can use the kinematic equation of motion (v^2-U^2)/2g = s. Where s is the maximum vertical displacement. I get 0.51m