Physics puzzler! Need serious help!!

[paigegail]

HELP!!!!! I am in grade 12 physics and my teacher gave us a puzzler for the weekend. I need serious help with it and all of my classmates cant figure it out either! The question is :

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?

Please help me if you can. How do I go about even starting the problem?

[Ambitwistor]

Work backwards. From how far away the box landed, you can find out what speed it had as it was leaving the cliff. From that speed and the distance it was pushed, you can find the net acceleration (and thus net force). From the net force and the force of friction, you can find Bob's force.

[paigegail]

I did what the first post said to do and I still can't get it. I end up with Fnet= 117.504 N. What the hell am I supposed to do? I've spent hours on this problem and it's driving me crazy!!

[Ambitwistor]

Obviously, you made some kind of error, but we can't determine what it is unless you show your work.