100=-4.9t^2+1000

Solution for 100=-4.9t^2+1000 equation:

100=-4.9t^2+1000

We move all terms to the left:

100-(-4.9t^2+1000)=0

We get rid of parentheses

4.9t^2-1000+100=0

We add all the numbers together, and all the variables

4.9t^2-900=0

a = 4.9; b = 0; c = -900;
Δ = b2-4ac
Δ = 02-4·4.9·(-900)
Δ = 17640
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:

$\sqrt{\Delta}=\sqrt{17640}=\sqrt{1764*10}=\sqrt{1764}*\sqrt{10}=42\sqrt{10}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-42\sqrt{10}}{2*4.9}=\frac{0-42\sqrt{10}}{9.8}
=-\frac{42\sqrt{10}}{9.8}
=-\frac{14\sqrt{10}}{3.2666666666667}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+42\sqrt{10}}{2*4.9}=\frac{0+42\sqrt{10}}{9.8}
=\frac{42\sqrt{10}}{9.8}
=\frac{14\sqrt{10}}{3.2666666666667}
$