How do i get the perfect square of:

3x^2+7x=-9

Did you mean "solve by completing the square " ?

if so, then first divide by 3

x^2 + (7/3)x = -3
x^2 + (7/3)x + 49/36 = -3 + 49/36
(x+7/6)^2 = -59/36
x+7/6 = ± √59 i /6
x = (-7 ± i√59)/6