12x+21-9x+21=28x2-175x+15

Solution for 12x+21-9x+21=28x2-175x+15 equation:

12x+21-9x+21=28x^2-175x+15

We move all terms to the left:

12x+21-9x+21-(28x^2-175x+15)=0

We add all the numbers together, and all the variables

3x-(28x^2-175x+15)+42=0

We get rid of parentheses

-28x^2+3x+175x-15+42=0

We add all the numbers together, and all the variables

-28x^2+178x+27=0

a = -28; b = 178; c = +27;
Δ = b2-4ac
Δ = 1782-4·(-28)·27
Δ = 34708
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:

$\sqrt{\Delta}=\sqrt{34708}=\sqrt{4*8677}=\sqrt{4}*\sqrt{8677}=2\sqrt{8677}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(178)-2\sqrt{8677}}{2*-28}=\frac{-178-2\sqrt{8677}}{-56}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(178)+2\sqrt{8677}}{2*-28}=\frac{-178+2\sqrt{8677}}{-56}
$