There are, perhaps, two ways to go about this. Use: W=F.s.cosθ

The other way is to use the equations of motion. Resolve the gravitational force so it is acting along the hill. Now you can get the resultant force acting on the sled, if you also resolve the force you pull with. Then use F=ma to get acceleration, and use the equations of motion.

The two methods are only superficially different, because we must remember the energy equations are derived (classically) from the equations of motion. But it is easier to solve it with the energy equations, as, and now we're going off topic, but they are more general, and the reason is a bit advanced, but if you cary on with physics, into university, you'll find out why.