Y=-0.005x^2+2.1x+21.5

Solution for Y=-0.005x^2+2.1x+21.5 equation:

=-0.005Y^2+2.1Y+21.5

We move all terms to the left:

-(-0.005Y^2+2.1Y+21.5)=0

We get rid of parentheses

0.005Y^2-2.1Y-21.5=0

a = 0.005; b = -2.1; c = -21.5;
Δ = b2-4ac
Δ = -2.12-4·0.005·(-21.5)
Δ = 4.84
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2.1)-\sqrt{4.84}}{2*0.005}=\frac{2.1-\sqrt{4.84}}{0.01}
$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2.1)+\sqrt{4.84}}{2*0.005}=\frac{2.1+\sqrt{4.84}}{0.01}
$