l*(l-4)=72

Solution for l*(l-4)=72 equation:

l*(l-4)=72

We move all terms to the left:

l*(l-4)-(72)=0

We multiply parentheses

l^2-4l-72=0

a = 1; b = -4; c = -72;
Δ = b2-4ac
Δ = -42-4·1·(-72)
Δ = 304
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$l_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$l_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{304}=\sqrt{16*19}=\sqrt{16}*\sqrt{19}=4\sqrt{19}$
$l_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-4\sqrt{19}}{2*1}=\frac{4-4\sqrt{19}}{2}
$
$l_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+4\sqrt{19}}{2*1}=\frac{4+4\sqrt{19}}{2}
$