21a^2-6=-39a

Solution for 21a^2-6=-39a equation:

21a^2-6=-39a

We move all terms to the left:

21a^2-6-(-39a)=0

We get rid of parentheses

21a^2+39a-6=0

a = 21; b = 39; c = -6;
Δ = b2-4ac
Δ = 392-4·21·(-6)
Δ = 2025
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{2025}=45$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(39)-45}{2*21}=\frac{-84}{42}
=-2
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(39)+45}{2*21}=\frac{6}{42}
=1/7
$