r(r+4)=780

Solution for r(r+4)=780 equation:

r(r+4)=780

We move all terms to the left:

r(r+4)-(780)=0

We multiply parentheses

r^2+4r-780=0

a = 1; b = 4; c = -780;
Δ = b2-4ac
Δ = 42-4·1·(-780)
Δ = 3136
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3136}=56$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(4)-56}{2*1}=\frac{-60}{2}
=-30
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(4)+56}{2*1}=\frac{52}{2}
=26
$