18r=-21r2

Solution for 18r=-21r2 equation:

18r=-21r^2

We move all terms to the left:

18r-(-21r^2)=0

We get rid of parentheses

21r^2+18r=0

a = 21; b = 18; c = 0;
Δ = b2-4ac
Δ = 182-4·21·0
Δ = 324
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{324}=18$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(18)-18}{2*21}=\frac{-36}{42}
=-6/7
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(18)+18}{2*21}=\frac{0}{42}
=0
$