-20=5t-4.9t^2

Solution for -20=5t-4.9t^2 equation:

-20=5t-4.9t^2

We move all terms to the left:

-20-(5t-4.9t^2)=0

We get rid of parentheses

4.9t^2-5t-20=0

a = 4.9; b = -5; c = -20;
Δ = b2-4ac
Δ = -52-4·4.9·(-20)
Δ = 417
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-5)-\sqrt{417}}{2*4.9}=\frac{5-\sqrt{417}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-5)+\sqrt{417}}{2*4.9}=\frac{5+\sqrt{417}}{9.8}
$