-4r(-2r)-6r+4r=20

Solution for -4r(-2r)-6r+4r=20 equation:

-4r(-2r)-6r+4r=20

We move all terms to the left:

-4r(-2r)-6r+4r-(20)=0

We add all the numbers together, and all the variables

-2r-4r(-2r)-20=0

We multiply parentheses

8r^2-2r-20=0

a = 8; b = -2; c = -20;
Δ = b2-4ac
Δ = -22-4·8·(-20)
Δ = 644
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{644}=\sqrt{4*161}=\sqrt{4}*\sqrt{161}=2\sqrt{161}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-2\sqrt{161}}{2*8}=\frac{2-2\sqrt{161}}{16}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+2\sqrt{161}}{2*8}=\frac{2+2\sqrt{161}}{16}
$