3b^2-9b=120

Solution for 3b^2-9b=120 equation:

3b^2-9b=120

We move all terms to the left:

3b^2-9b-(120)=0

a = 3; b = -9; c = -120;
Δ = b2-4ac
Δ = -92-4·3·(-120)
Δ = 1521
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1521}=39$
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-39}{2*3}=\frac{-30}{6}
=-5
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+39}{2*3}=\frac{48}{6}
=8
$