-30=-12t-4.9t^2

Solution for -30=-12t-4.9t^2 equation:

-30=-12t-4.9t^2

We move all terms to the left:

-30-(-12t-4.9t^2)=0

We get rid of parentheses

4.9t^2+12t-30=0

a = 4.9; b = 12; c = -30;
Δ = b2-4ac
Δ = 122-4·4.9·(-30)
Δ = 732
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:

$\sqrt{\Delta}=\sqrt{732}=\sqrt{4*183}=\sqrt{4}*\sqrt{183}=2\sqrt{183}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(12)-2\sqrt{183}}{2*4.9}=\frac{-12-2\sqrt{183}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(12)+2\sqrt{183}}{2*4.9}=\frac{-12+2\sqrt{183}}{9.8}
$