How would I go about solving this?
You don't. That is not a "one to one" function and so does not have an inverse. If is the inverse of f(x), then we must have , that is, must 'undo' whatever f does. But f(6)= 0 and f(-1)= 0. can't be both 6 and -1!
Luke774, for some reason, chose to take only the "+" sign on the square root. That's valid but gives the inverse of a slightly different function: with and undefined for x< 5/2. Same formula but different domain so different function.