0=72+15x-x^2

Solution for 0=72+15x-x^2 equation:

0=72+15x-x^2

We move all terms to the left:

0-(72+15x-x^2)=0

We add all the numbers together, and all the variables

-(72+15x-x^2)=0

We get rid of parentheses

x^2-15x-72=0

a = 1; b = -15; c = -72;
Δ = b2-4ac
Δ = -152-4·1·(-72)
Δ = 513
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{513}=\sqrt{9*57}=\sqrt{9}*\sqrt{57}=3\sqrt{57}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-3\sqrt{57}}{2*1}=\frac{15-3\sqrt{57}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+3\sqrt{57}}{2*1}=\frac{15+3\sqrt{57}}{2}
$