4.9t2-9.8t-39.2=0

Solution for 4.9t2-9.8t-39.2=0 equation:

4.9t^2-9.8t-39.2=0

a = 4.9; b = -9.8; c = -39.2;
Δ = b2-4ac
Δ = -9.82-4·4.9·(-39.2)
Δ = 864.36
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{-(-9.8)-\sqrt{864.36}}{2*4.9}=\frac{9.8-\sqrt{864.36}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9.8)+\sqrt{864.36}}{2*4.9}=\frac{9.8+\sqrt{864.36}}{9.8}
$