200=4.9*t^2

Solution for 200=4.9*t^2 equation:

200=4.9t^2

We move all terms to the left:

200-(4.9t^2)=0

We get rid of parentheses

-4.9t^2+200=0

a = -4.9; b = 0; c = +200;
Δ = b2-4ac
Δ = 02-4·(-4.9)·200
Δ = 3920
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:

$\sqrt{\Delta}=\sqrt{3920}=\sqrt{16*245}=\sqrt{16}*\sqrt{245}=4\sqrt{245}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-4\sqrt{245}}{2*-4.9}=\frac{0-4\sqrt{245}}{-9.8}
=-\frac{4\sqrt{245}}{-9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+4\sqrt{245}}{2*-4.9}=\frac{0+4\sqrt{245}}{-9.8}
=\frac{4\sqrt{245}}{-9.8}
$