81=x2+5.25x

Solution for 81=x2+5.25x equation:

81=x2+5.25x

We move all terms to the left:

81-(x2+5.25x)=0

We add all the numbers together, and all the variables

-(+x^2+5.25x)+81=0

We get rid of parentheses

-x^2-5.25x+81=0

We add all the numbers together, and all the variables

-1x^2-5.25x+81=0

a = -1; b = -5.25; c = +81;
Δ = b2-4ac
Δ = -5.252-4·(-1)·81
Δ = 351.5625
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-5.25)-\sqrt{351.5625}}{2*-1}=\frac{5.25-\sqrt{351.5625}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-5.25)+\sqrt{351.5625}}{2*-1}=\frac{5.25+\sqrt{351.5625}}{-2}
$