X^2=-20x+21

Solution for X^2=-20x+21 equation:

X^2=-20X+21

We move all terms to the left:

X^2-(-20X+21)=0

We get rid of parentheses

X^2+20X-21=0

a = 1; b = 20; c = -21;
Δ = b2-4ac
Δ = 202-4·1·(-21)
Δ = 484
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{484}=22$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-22}{2*1}=\frac{-42}{2}
=-21
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+22}{2*1}=\frac{2}{2}
=1
$