X(x+3)=720

Solution for X(x+3)=720 equation:

X(X+3)=720

We move all terms to the left:

X(X+3)-(720)=0

We multiply parentheses

X^2+3X-720=0

a = 1; b = 3; c = -720;
Δ = b2-4ac
Δ = 32-4·1·(-720)
Δ = 2889
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:

$\sqrt{\Delta}=\sqrt{2889}=\sqrt{9*321}=\sqrt{9}*\sqrt{321}=3\sqrt{321}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(3)-3\sqrt{321}}{2*1}=\frac{-3-3\sqrt{321}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(3)+3\sqrt{321}}{2*1}=\frac{-3+3\sqrt{321}}{2}
$