21=24.5t-4.9t^2

Solution for 21=24.5t-4.9t^2 equation:

21=24.5t-4.9t^2

We move all terms to the left:

21-(24.5t-4.9t^2)=0

We get rid of parentheses

4.9t^2-24.5t+21=0

a = 4.9; b = -24.5; c = +21;
Δ = b2-4ac
Δ = -24.52-4·4.9·21
Δ = 188.65
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-24.5)-\sqrt{188.65}}{2*4.9}=\frac{24.5-\sqrt{188.65}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-24.5)+\sqrt{188.65}}{2*4.9}=\frac{24.5+\sqrt{188.65}}{9.8}
$