We are given y as a function of x, let's first compute dy/dx, and then solve the equation dy/dx =0. dy/dx = 9 -1/x2. Then dy/dx = 0 is equivalent to 9 = 1/x2. Taking x2 on the LHS and 9 on the RHS we obtain x2 = 1/9. Finally, the two values of x are -1/3 or 1/3.