3r^2=21r

Solution for 3r^2=21r equation:

3r^2=21r

We move all terms to the left:

3r^2-(21r)=0

a = 3; b = -21; c = 0;
Δ = b2-4ac
Δ = -212-4·3·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-21}{2*3}=\frac{0}{6}
=0
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+21}{2*3}=\frac{42}{6}
=7
$