-4c^2-20c-7=0

Solution for -4c^2-20c-7=0 equation:

-4c^2-20c-7=0

a = -4; b = -20; c = -7;
Δ = b2-4ac
Δ = -202-4·(-4)·(-7)
Δ = 288
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{288}=\sqrt{144*2}=\sqrt{144}*\sqrt{2}=12\sqrt{2}$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-12\sqrt{2}}{2*-4}=\frac{20-12\sqrt{2}}{-8}
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+12\sqrt{2}}{2*-4}=\frac{20+12\sqrt{2}}{-8}
$