2x+x(x+10)=100

Solution for 2x+x(x+10)=100 equation:

2x+x(x+10)=100

We move all terms to the left:

2x+x(x+10)-(100)=0

We multiply parentheses

x^2+2x+10x-100=0

We add all the numbers together, and all the variables

x^2+12x-100=0

a = 1; b = 12; c = -100;
Δ = b2-4ac
Δ = 122-4·1·(-100)
Δ = 544
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:

$\sqrt{\Delta}=\sqrt{544}=\sqrt{16*34}=\sqrt{16}*\sqrt{34}=4\sqrt{34}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(12)-4\sqrt{34}}{2*1}=\frac{-12-4\sqrt{34}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(12)+4\sqrt{34}}{2*1}=\frac{-12+4\sqrt{34}}{2}
$