0=3x^2+21x

Solution for 0=3x^2+21x equation:

0=3x^2+21x

We move all terms to the left:

0-(3x^2+21x)=0

We add all the numbers together, and all the variables

-(3x^2+21x)=0

We get rid of parentheses

-3x^2-21x=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:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

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