Take half of the coefficient of x: It is 3, and half that is 3/2.
Then x^2+3x=6 becomes:
x^2+3x + (3/2)^2 =6 + (3/2)^2, and
(x+3/2)^2 = 6 + 9/4
You were not asked to solve the equation, but why not do it for the practice?
Solve (x+3/2)^2 = 6 + 9/4 for x. There will be 2 values.