(X-3)(x-10)=120

Solution for (X-3)(x-10)=120 equation:

(X-3)(X-10)=120

We move all terms to the left:

(X-3)(X-10)-(120)=0

We multiply parentheses ..

(+X^2-10X-3X+30)-120=0

We get rid of parentheses

X^2-10X-3X+30-120=0

We add all the numbers together, and all the variables

X^2-13X-90=0

a = 1; b = -13; c = -90;
Δ = b2-4ac
Δ = -132-4·1·(-90)
Δ = 529
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{529}=23$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-13)-23}{2*1}=\frac{-10}{2}
=-5
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-13)+23}{2*1}=\frac{36}{2}
=18
$