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

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

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

We move all terms to the left:

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

We get rid of parentheses

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

We add all the numbers together, and all the variables

9.8t^2+15t=0

a = 9.8; b = 15; c = 0;
Δ = b2-4ac
Δ = 152-4·9.8·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-15}{2*9.8}=\frac{-30}{19.6}
=-1+7.4285714285714/14
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+15}{2*9.8}=\frac{0}{19.6}
=0
$