-5a(a+4)=15

Solution for -5a(a+4)=15 equation:

-5a(a+4)=15

We move all terms to the left:

-5a(a+4)-(15)=0

We multiply parentheses

-5a^2-20a-15=0

a = -5; b = -20; c = -15;
Δ = b2-4ac
Δ = -202-4·(-5)·(-15)
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-10}{2*-5}=\frac{10}{-10}
=-1
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+10}{2*-5}=\frac{30}{-10}
=-3
$