-120=27.9t-4.9t2

Solution for -120=27.9t-4.9t2 equation:

-120=27.9t-4.9t^2

We move all terms to the left:

-120-(27.9t-4.9t^2)=0

We get rid of parentheses

4.9t^2-27.9t-120=0

a = 4.9; b = -27.9; c = -120;
Δ = b2-4ac
Δ = -27.92-4·4.9·(-120)
Δ = 3130.41
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{-(-27.9)-\sqrt{3130.41}}{2*4.9}=\frac{27.9-\sqrt{3130.41}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-27.9)+\sqrt{3130.41}}{2*4.9}=\frac{27.9+\sqrt{3130.41}}{9.8}
$