x(8=7)-5x+21=31

Solution for x(8=7)-5x+21=31 equation:

x(8=7)-5x+21=31

We move all terms to the left:

x(8-(7)-5x+21)=0

We add all the numbers together, and all the variables

x(-5x+22)=0

We multiply parentheses

-5x^2+22x=0

a = -5; b = 22; c = 0;
Δ = b2-4ac
Δ = 222-4·(-5)·0
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{-(22)-22}{2*-5}=\frac{-44}{-10}
=4+2/5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(22)+22}{2*-5}=\frac{0}{-10}
=0
$