Find the x-value of all points where the function has relative extrema. Find the values of any relative extrema.

f(x)=x^3-12x+2

y'=3x^2-12=0

y'=3x^2=12

y'=x^2=4

Now after this point what i did was take the square root of each side. Now aren't i supposed to end up with +/- 2 and find the matching y-values? I get:

Max:0@-2

Min:0@2

But the computer say this is wrong i know that 0@2 is right but where did i go wrong?