A 1.90 kg box slides down a rough incline plane from a height h of 1.51 m. The box had a speed of 2.99 m/s at the top and a speed of 2.56 m/s at the bottom. Calculate the mechanical energy lost due to friction (as heat, etc.).
Welcome to PF. :) Hopefully you'll be wanting to change that username soon enough.
Look at the total initial energy, and at the total final energy, remember that conservation of tells you that they're equal. You have three energies involved in this problem. Potential gravitational energy, kinetic energy and the work of friction, try and see which ones you know for the initial and final states, and which ones you don't.