84x=21x2

Solution for 84x=21x2 equation:

84x=21x^2

We move all terms to the left:

84x-(21x^2)=0

determiningTheFunctionDomain
-21x^2+84x=0

a = -21; b = 84; c = 0;
Δ = b2-4ac
Δ = 842-4·(-21)·0
Δ = 7056
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{7056}=84$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(84)-84}{2*-21}=\frac{-168}{-42}
=+4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(84)+84}{2*-21}=\frac{0}{-42}
=0
$