-5r(4-r)=25

Solution for -5r(4-r)=25 equation:

-5r(4-r)=25

We move all terms to the left:

-5r(4-r)-(25)=0

We add all the numbers together, and all the variables

-5r(-1r+4)-25=0

We multiply parentheses

5r^2-20r-25=0

a = 5; b = -20; c = -25;
Δ = b2-4ac
Δ = -202-4·5·(-25)
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-30}{2*5}=\frac{-10}{10}
=-1
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+30}{2*5}=\frac{50}{10}
=5
$