-15=y^2+8y

Solution for -15=y^2+8y equation:

-15=y^2+8y

We move all terms to the left:

-15-(y^2+8y)=0

We get rid of parentheses

-y^2-8y-15=0

We add all the numbers together, and all the variables

-1y^2-8y-15=0

a = -1; b = -8; c = -15;
Δ = b2-4ac
Δ = -82-4·(-1)·(-15)
Δ = 4
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{4}=2$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-2}{2*-1}=\frac{6}{-2}
=-3
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+2}{2*-1}=\frac{10}{-2}
=-5
$