Step by step
    (x + 6)^2 = 36
Take square root of both sides
    x+6 = +/- 6.
Consider two cases in succession
    (a)  x+6 = 6  --->    x= 0.
    (b)  x+6 = -6  --->  x= -12.
ANSWER.  The two solutions are -12 and 0.