21.3=-4x^2+21.3x

Solution for 21.3=-4x^2+21.3x equation:

21.3=-4x^2+21.3x

We move all terms to the left:

21.3-(-4x^2+21.3x)=0

We get rid of parentheses

4x^2-21.3x+21.3=0

a = 4; b = -21.3; c = +21.3;
Δ = b2-4ac
Δ = -21.32-4·4·21.3
Δ = 112.89
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21.3)-\sqrt{112.89}}{2*4}=\frac{21.3-\sqrt{112.89}}{8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21.3)+\sqrt{112.89}}{2*4}=\frac{21.3+\sqrt{112.89}}{8}
$