0=405-168x+12x^2

Solution for 0=405-168x+12x^2 equation:

0=405-168x+12x^2

We move all terms to the left:

0-(405-168x+12x^2)=0

We add all the numbers together, and all the variables

-(405-168x+12x^2)=0

We get rid of parentheses

-12x^2+168x-405=0

a = -12; b = 168; c = -405;
Δ = b2-4ac
Δ = 1682-4·(-12)·(-405)
Δ = 8784
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{8784}=\sqrt{144*61}=\sqrt{144}*\sqrt{61}=12\sqrt{61}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(168)-12\sqrt{61}}{2*-12}=\frac{-168-12\sqrt{61}}{-24}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(168)+12\sqrt{61}}{2*-12}=\frac{-168+12\sqrt{61}}{-24}
$