840=2.1z^2

Solution for 840=2.1z^2 equation:

840=2.1z^2

We move all terms to the left:

840-(2.1z^2)=0

We get rid of parentheses

-2.1z^2+840=0

a = -2.1; b = 0; c = +840;
Δ = b2-4ac
Δ = 02-4·(-2.1)·840
Δ = 7056
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{7056}=84$
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-84}{2*-2.1}=\frac{-84}{-4.2}
=+20
$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+84}{2*-2.1}=\frac{84}{-4.2}
=-20
$