18=r^2+3r

Solution for 18=r^2+3r equation:

18=r^2+3r

We move all terms to the left:

18-(r^2+3r)=0

We get rid of parentheses

-r^2-3r+18=0

We add all the numbers together, and all the variables

-1r^2-3r+18=0

a = -1; b = -3; c = +18;
Δ = b2-4ac
Δ = -32-4·(-1)·18
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{81}=9$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-3)-9}{2*-1}=\frac{-6}{-2}
=+3
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3)+9}{2*-1}=\frac{12}{-2}
=-6
$