21a=7a^2

Solution for 21a=7a^2 equation:

21a=7a^2

We move all terms to the left:

21a-(7a^2)=0

determiningTheFunctionDomain
-7a^2+21a=0

a = -7; b = 21; c = 0;
Δ = b2-4ac
Δ = 212-4·(-7)·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-21}{2*-7}=\frac{-42}{-14}
=+3
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+21}{2*-7}=\frac{0}{-14}
=0
$