you are trying to what... CTS?
Foe such questions adopt the folloing algorithm.
Step - 1: Make coefficient = 1. in this case : divide by 4: x^2 + 6x - 9/2 = 0
step - 2: transfer the constant term to the RHS: we will get x^2 + 6x = 9/2
Step 3: Add the square of half the coefficient of x on both the sides of the equation. in this case the coefficient of x = 6; Half the coefficient of x = 3 and square of this is 9. Thus we have: x^2 + 6x + 9= 9/2 + 9
That gives ( x + 3 )^2 = 27/2