90=100-4.9t^2

Solution for 90=100-4.9t^2 equation:

90=100-4.9t^2

We move all terms to the left:

90-(100-4.9t^2)=0

We get rid of parentheses

4.9t^2-100+90=0

We add all the numbers together, and all the variables

4.9t^2-10=0

a = 4.9; b = 0; c = -10;
Δ = b2-4ac
Δ = 02-4·4.9·(-10)
Δ = 196
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{196}=14$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-14}{2*4.9}=\frac{-14}{9.8}
=-1+3.5/8.16666666667
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+14}{2*4.9}=\frac{14}{9.8}
=1+3.5/8.16666666667
$