4.9t2-15t-20=0

Solution for 4.9t2-15t-20=0 equation:

4.9t^2-15t-20=0

a = 4.9; b = -15; c = -20;
Δ = b2-4ac
Δ = -152-4·4.9·(-20)
Δ = 617
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{-(-15)-\sqrt{617}}{2*4.9}=\frac{15-\sqrt{617}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+\sqrt{617}}{2*4.9}=\frac{15+\sqrt{617}}{9.8}
$