(X+1)(x-18)=0

Solution for (X+1)(x-18)=0 equation:

(X+1)(X-18)=0

We multiply parentheses ..

(+X^2-18X+X-18)=0

We get rid of parentheses

X^2-18X+X-18=0

We add all the numbers together, and all the variables

X^2-17X-18=0

a = 1; b = -17; c = -18;
Δ = b2-4ac
Δ = -172-4·1·(-18)
Δ = 361
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{361}=19$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-17)-19}{2*1}=\frac{-2}{2}
=-1
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-17)+19}{2*1}=\frac{36}{2}
=18
$