The roots of this function are the extrema of the original function. That is, set and solve (hint recognize that 0 must be a root, so to find others, solve .)
Take the next derivative and substitute in the critical points found above. If the result is negative then it's a max. If it's positive then it's a min. If it equals zero, then the point is neither max or min (throw it out), unless the whole original function is constant, which it is not in this case.