100=15t^2+9.8t

Solution for 100=15t^2+9.8t equation:

100=15t^2+9.8t

We move all terms to the left:

100-(15t^2+9.8t)=0

We get rid of parentheses

-15t^2-9.8t+100=0

a = -15; b = -9.8; c = +100;
Δ = b2-4ac
Δ = -9.82-4·(-15)·100
Δ = 6096.04
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{-(-9.8)-\sqrt{6096.04}}{2*-15}=\frac{9.8-\sqrt{6096.04}}{-30}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9.8)+\sqrt{6096.04}}{2*-15}=\frac{9.8+\sqrt{6096.04}}{-30}
$