-7n(3n+3)=0

Solution for -7n(3n+3)=0 equation:

-7n(3n+3)=0

We multiply parentheses

-21n^2-21n=0

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

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