X2+91=20x

Solution for X2+91=20x equation:

X2+91=20X

We move all terms to the left:

X2+91-(20X)=0

We add all the numbers together, and all the variables

X^2-20X+91=0

a = 1; b = -20; c = +91;
Δ = b2-4ac
Δ = -202-4·1·91
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-6}{2*1}=\frac{14}{2}
=7
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+6}{2*1}=\frac{26}{2}
=13
$