x^2+x(x+10)=720

Solution for x^2+x(x+10)=720 equation:

x^2+x(x+10)=720

We move all terms to the left:

x^2+x(x+10)-(720)=0

We multiply parentheses

x^2+x^2+10x-720=0

We add all the numbers together, and all the variables

2x^2+10x-720=0

a = 2; b = 10; c = -720;
Δ = b2-4ac
Δ = 102-4·2·(-720)
Δ = 5860
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:

$\sqrt{\Delta}=\sqrt{5860}=\sqrt{4*1465}=\sqrt{4}*\sqrt{1465}=2\sqrt{1465}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(10)-2\sqrt{1465}}{2*2}=\frac{-10-2\sqrt{1465}}{4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(10)+2\sqrt{1465}}{2*2}=\frac{-10+2\sqrt{1465}}{4}
$