30=18t+9.8t^2

Solution for 30=18t+9.8t^2 equation:

30=18t+9.8t^2

We move all terms to the left:

30-(18t+9.8t^2)=0

We get rid of parentheses

-9.8t^2-18t+30=0

a = -9.8; b = -18; c = +30;
Δ = b2-4ac
Δ = -182-4·(-9.8)·30
Δ = 1500
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:

$\sqrt{\Delta}=\sqrt{1500}=\sqrt{100*15}=\sqrt{100}*\sqrt{15}=10\sqrt{15}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-18)-10\sqrt{15}}{2*-9.8}=\frac{18-10\sqrt{15}}{-19.6}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-18)+10\sqrt{15}}{2*-9.8}=\frac{18+10\sqrt{15}}{-19.6}
$