Answer:
x=6, x=-2
Explanation:
In order to complete the square of this problem, we add 4, and also subtract 4, because 4 wasn't there, so we have to take away what was added:
x^2-4x+4-4-12=0
x^2-4x+4=(x-2)^2
(x-2)^2-16=0
(x-2)^2=16

x-2=±4
x-2=4
x=6
x-2=-4
x=-2
Thus, x=6 and x=-2