4c(c-5)=20

Solution for 4c(c-5)=20 equation:

4c(c-5)=20

We move all terms to the left:

4c(c-5)-(20)=0

We multiply parentheses

4c^2-20c-20=0

a = 4; b = -20; c = -20;
Δ = b2-4ac
Δ = -202-4·4·(-20)
Δ = 720
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:

$\sqrt{\Delta}=\sqrt{720}=\sqrt{144*5}=\sqrt{144}*\sqrt{5}=12\sqrt{5}$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-12\sqrt{5}}{2*4}=\frac{20-12\sqrt{5}}{8}
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+12\sqrt{5}}{2*4}=\frac{20+12\sqrt{5}}{8}
$