H(x)=20-4.9x^2

Solution for H(x)=20-4.9x^2 equation:

(H)=20-4.9H^2

We move all terms to the left:

(H)-(20-4.9H^2)=0

We get rid of parentheses

4.9H^2+H-20=0

a = 4.9; b = 1; c = -20;
Δ = b2-4ac
Δ = 12-4·4.9·(-20)
Δ = 393
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$H_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$H_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$H_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{393}}{2*4.9}=\frac{-1-\sqrt{393}}{9.8}
$
$H_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{393}}{2*4.9}=\frac{-1+\sqrt{393}}{9.8}
$