Looks like you need to know how to calculate the derivative, how to model with polynomials, and the relationship between derivative and acceleration. More specifically, you need to know what it would mean for the function P(x) to represent height h at distance l, and height 0 at distance 0. You should also use the hint about putting a restriction on P'(x). There's a nice property that you want the graph to have near the origin and at the start of descent. Looks like you'll need to solve a system of equations at some point, too.