126=15x^2-x^2

Solution for 126=15x^2-x^2 equation:

126=15x^2-x^2

We move all terms to the left:

126-(15x^2-x^2)=0

We get rid of parentheses

-15x^2+x^2+126=0

We add all the numbers together, and all the variables

-14x^2+126=0

a = -14; b = 0; c = +126;
Δ = b2-4ac
Δ = 02-4·(-14)·126
Δ = 7056
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{7056}=84$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-84}{2*-14}=\frac{-84}{-28}
=+3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+84}{2*-14}=\frac{84}{-28}
=-3
$