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**HELP!! Projectile motion and Force puzzler!!**

A 10.8 kg box is 5.0 m from the edge of a 12.96 m high cliff. Bob pushes the box over the edge of the cliff by applying a constant horizontal force to it until the box goes over the edge. The force of friction on the box is 49.0 N and the box lands 7.5 m from the base of the cliff. How much force did Bob apply to the box while he was pushing?

What am i supposed to do... I know I'm supposed to work backwards...which is what I did. I found the velocities in the vertical and horizontal directions...what do I do now?!