4.9t^2-7.9t-1.8=0

Solution for 4.9t^2-7.9t-1.8=0 equation:

4.9t^2-7.9t-1.8=0

a = 4.9; b = -7.9; c = -1.8;
Δ = b2-4ac
Δ = -7.92-4·4.9·(-1.8)
Δ = 97.69
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{-(-7.9)-\sqrt{97.69}}{2*4.9}=\frac{7.9-\sqrt{97.69}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7.9)+\sqrt{97.69}}{2*4.9}=\frac{7.9+\sqrt{97.69}}{9.8}
$