x^2+12x+6=0 subtract 6 from both sides...
x^2+12x=-6 halve linear coefficient, square it, and add it to both sides...
x^2+12x+36=30 now the left side is a perfect square...
(x+6)^2=30 take square root of both sides...
x+6=30^(1/2) subtract 6 from both sides...
x=-6-30^(1/2) and -6+30^(1/2)