12=3x^2+9x

Solution for 12=3x^2+9x equation:

12=3x^2+9x

We move all terms to the left:

12-(3x^2+9x)=0

We get rid of parentheses

-3x^2-9x+12=0

a = -3; b = -9; c = +12;
Δ = b2-4ac
Δ = -92-4·(-3)·12
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-15}{2*-3}=\frac{-6}{-6}
=1
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+15}{2*-3}=\frac{24}{-6}
=-4
$