-x2+8x=-15

Solution for -x2+8x=-15 equation:

-x2+8x=-15

We move all terms to the left:

-x2+8x-(-15)=0

We add all the numbers together, and all the variables

-1x^2+8x+15=0

a = -1; b = 8; c = +15;
Δ = b2-4ac
Δ = 82-4·(-1)·15
Δ = 124
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:

$\sqrt{\Delta}=\sqrt{124}=\sqrt{4*31}=\sqrt{4}*\sqrt{31}=2\sqrt{31}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(8)-2\sqrt{31}}{2*-1}=\frac{-8-2\sqrt{31}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(8)+2\sqrt{31}}{2*-1}=\frac{-8+2\sqrt{31}}{-2}
$