Sketch of solution. Let P(x)=a0+a1x+...+akxk. Both sides of the equation contains a polynomial of n, degree is k. The coefficients must pairwise match.
Since the coefficient of nk is on the left-hand side, and it is ak on the right-hand side. Therefore, .
If then we have a homogenous system of linear equations for the coefficients and one of these equations vanishes. So the system has infinitely many solutions.
For =2 we obtain k=3 and P(x)=c(x3-x).