-3(5c+7);c=6

Solution for -3(5c+7);c=6 equation:

-3(5c+7)c=6

We move all terms to the left:

-3(5c+7)c-(6)=0

We multiply parentheses

-15c^2-21c-6=0

a = -15; b = -21; c = -6;
Δ = b2-4ac
Δ = -212-4·(-15)·(-6)
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{81}=9$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-9}{2*-15}=\frac{12}{-30}
=-2/5
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+9}{2*-15}=\frac{30}{-30}
=-1
$