-3a(a-21)=12

Solution for -3a(a-21)=12 equation:

-3a(a-21)=12

We move all terms to the left:

-3a(a-21)-(12)=0

We multiply parentheses

-3a^2+63a-12=0

a = -3; b = 63; c = -12;
Δ = b2-4ac
Δ = 632-4·(-3)·(-12)
Δ = 3825
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{3825}=\sqrt{225*17}=\sqrt{225}*\sqrt{17}=15\sqrt{17}$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(63)-15\sqrt{17}}{2*-3}=\frac{-63-15\sqrt{17}}{-6}
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(63)+15\sqrt{17}}{2*-3}=\frac{-63+15\sqrt{17}}{-6}
$