Shall I go on? Or can you take it from here...with these hints:
"Width" is determined by 'solving' the equation.
"Volume" is determined by the 'area under the curve' (which should come from taking the integral between 0 and your 'width') multiplied by the depth (which looks to be 500 ft).
If you really want to get fancy and turn your answer in with how 'volume' would be expressesd on a real construction site, the solution should be in cubic yards (which is # of cubed feet divided by 27...or 3 cubed).
For example: 100 ft^3 = 3.7cy
And for what it's worth, a typical dump truck holds 10 cy.
(and I hope I'm right
. It's been a looooong time.)