math problem

Solve for x:
1/2 x (x+6) = 36

Multiply both sides by 2:
x (x+6) = 72

Expand out terms of the left hand side:
x^2+6 x = 72

Add 9 to both sides:
x^2+6 x+9 = 81

Write the left hand side as a square:
(x+3)^2 = 81

Take the square root of both sides:
x+3 = 9 or x+3 = -9

Subtract 3 from both sides:
x = 6 or x+3 = -9

Subtract 3 from both sides:
Answer: | x = 6                        or x = -12

$(1/2)(x)(6+x)=36$

$(x)(6+x)=72$

$x^2+6x-72=0$

$(x+12)(x-6)=0$

$x=-12\space or\space x=6$