100=2.16*t^2

Solution for 100=2.16*t^2 equation:

100=2.16t^2

We move all terms to the left:

100-(2.16t^2)=0

We get rid of parentheses

-2.16t^2+100=0

a = -2.16; b = 0; c = +100;
Δ = b2-4ac
Δ = 02-4·(-2.16)·100
Δ = 864
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:

$\sqrt{\Delta}=\sqrt{864}=\sqrt{144*6}=\sqrt{144}*\sqrt{6}=12\sqrt{6}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-12\sqrt{6}}{2*-2.16}=\frac{0-12\sqrt{6}}{-4.32}
=-\frac{12\sqrt{6}}{-4.32}
=-\frac{3\sqrt{6}}{-1.08}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+12\sqrt{6}}{2*-2.16}=\frac{0+12\sqrt{6}}{-4.32}
=\frac{12\sqrt{6}}{-4.32}
=\frac{3\sqrt{6}}{-1.08}
$