(X+10)x=90

Solution for (X+10)x=90 equation:

(X+10)X=90

We move all terms to the left:

(X+10)X-(90)=0

We multiply parentheses

X^2+10X-90=0

a = 1; b = 10; c = -90;
Δ = b2-4ac
Δ = 102-4·1·(-90)
Δ = 460
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:

$\sqrt{\Delta}=\sqrt{460}=\sqrt{4*115}=\sqrt{4}*\sqrt{115}=2\sqrt{115}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(10)-2\sqrt{115}}{2*1}=\frac{-10-2\sqrt{115}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(10)+2\sqrt{115}}{2*1}=\frac{-10+2\sqrt{115}}{2}
$