0=100-4.9t^2

Solution for 0=100-4.9t^2 equation:

0=100-4.9t^2

We move all terms to the left:

0-(100-4.9t^2)=0

We add all the numbers together, and all the variables

-(100-4.9t^2)=0

We get rid of parentheses

4.9t^2-100=0

a = 4.9; b = 0; c = -100;
Δ = b2-4ac
Δ = 02-4·4.9·(-100)
Δ = 1960
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:

$\sqrt{\Delta}=\sqrt{1960}=\sqrt{4*490}=\sqrt{4}*\sqrt{490}=2\sqrt{490}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{490}}{2*4.9}=\frac{0-2\sqrt{490}}{9.8}
=-\frac{2\sqrt{490}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{490}}{2*4.9}=\frac{0+2\sqrt{490}}{9.8}
=\frac{2\sqrt{490}}{9.8}
$