Y=-4.9x^2+21x

Solution for Y=-4.9x^2+21x equation:

=-4.9Y^2+21Y

We move all terms to the left:

-(-4.9Y^2+21Y)=0

We get rid of parentheses

4.9Y^2-21Y=0

a = 4.9; b = -21; c = 0;
Δ = b2-4ac
Δ = -212-4·4.9·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-21}{2*4.9}=\frac{0}{9.8}
=0
$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+21}{2*4.9}=\frac{42}{9.8}
=4+2.8/9.8
$