X^2-21x=100

Solution for X^2-21x=100 equation:

X^2-21X=100

We move all terms to the left:

X^2-21X-(100)=0

a = 1; b = -21; c = -100;
Δ = b2-4ac
Δ = -212-4·1·(-100)
Δ = 841
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{841}=29$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-29}{2*1}=\frac{-8}{2}
=-4
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+29}{2*1}=\frac{50}{2}
=25
$