You're asking far too many questions. It would do no good for us to help you if you honestly don't know what you're doing with any of them.
I want you to think this problem through and we'll build on it.
work = force * distance
They seem to want you to assume a lot. Not really sure why this is even a calc question.
On a force-distance graph, y = 750 when x = 3 and y = 0 when x = 0.
The integral of a force-distance graph gives you force*distance, or work.
So we have
And we want
We know that W(0) is 0, so W(3) must be 2250.