-300=-8x^2-15x

Solution for -300=-8x^2-15x equation:

-300=-8x^2-15x

We move all terms to the left:

-300-(-8x^2-15x)=0

We get rid of parentheses

8x^2+15x-300=0

a = 8; b = 15; c = -300;
Δ = b2-4ac
Δ = 152-4·8·(-300)
Δ = 9825
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:

$\sqrt{\Delta}=\sqrt{9825}=\sqrt{25*393}=\sqrt{25}*\sqrt{393}=5\sqrt{393}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-5\sqrt{393}}{2*8}=\frac{-15-5\sqrt{393}}{16}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+5\sqrt{393}}{2*8}=\frac{-15+5\sqrt{393}}{16}
$