-30=50t-4.9t^2

Solution for -30=50t-4.9t^2 equation:

-30=50t-4.9t^2

We move all terms to the left:

-30-(50t-4.9t^2)=0

We get rid of parentheses

4.9t^2-50t-30=0

a = 4.9; b = -50; c = -30;
Δ = b2-4ac
Δ = -502-4·4.9·(-30)
Δ = 3088
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{3088}=\sqrt{16*193}=\sqrt{16}*\sqrt{193}=4\sqrt{193}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-50)-4\sqrt{193}}{2*4.9}=\frac{50-4\sqrt{193}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-50)+4\sqrt{193}}{2*4.9}=\frac{50+4\sqrt{193}}{9.8}
$