84=x2+8x

Solution for 84=x2+8x equation:

84=x2+8x

We move all terms to the left:

84-(x2+8x)=0

We add all the numbers together, and all the variables

-(+x^2+8x)+84=0

We get rid of parentheses

-x^2-8x+84=0

We add all the numbers together, and all the variables

-1x^2-8x+84=0

a = -1; b = -8; c = +84;
Δ = b2-4ac
Δ = -82-4·(-1)·84
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-20}{2*-1}=\frac{-12}{-2}
=+6
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+20}{2*-1}=\frac{28}{-2}
=-14
$