Energy, work and power — practice question

A lorry with mass $15\,000\,\text{kg}$ travels up a hill that is $500\,\text{m}$ long at constant speed. The slope is at an angle of $2.5^\circ$ to the horizontal. The resistance to the lorry’s motion stays constant at $800\,\text{N}$. On the way back, the lorry arrives at the top of the hill with speed $20\,\text{m s}^{-1}$ and moves downhill with a constant driving force of $2000\,\text{N}$. The resistance to the lorry’s motion is again constant at $800\,\text{N}$.