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

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

=-9.8D^2-15D+100

We move all terms to the left:

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

We get rid of parentheses

9.8D^2+15D-100=0

a = 9.8; b = 15; c = -100;
Δ = b2-4ac
Δ = 152-4·9.8·(-100)
Δ = 4145
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-\sqrt{4145}}{2*9.8}=\frac{-15-\sqrt{4145}}{19.6}
$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+\sqrt{4145}}{2*9.8}=\frac{-15+\sqrt{4145}}{19.6}
$