Solve this quadratic equation by completing the square. x^2 + 6x = 18

Take the b term, in this case 6, divide it by 2 (now you have 3), and then square that quotient (9). Now add that result to both sides:

x² + 6x + 9 = 18 + 9
(x + 3)² = 27

Square root both sides so you are left with only (x + 3) on the left side. 

√(x + 3)² = √27
x + 3 = +/- √27

Simplify the √27:
x + 3 = +/- 3√3

Subtract 3 from both sides to isolate variable x:
x = - 3 +/- 3√3

Your answers are:
x = - 3 - 3√3 and x = - 3+ 3√3