-21=5.4t-4.9t^2

Solution for -21=5.4t-4.9t^2 equation:

-21=5.4t-4.9t^2

We move all terms to the left:

-21-(5.4t-4.9t^2)=0

We get rid of parentheses

4.9t^2-5.4t-21=0

a = 4.9; b = -5.4; c = -21;
Δ = b2-4ac
Δ = -5.42-4·4.9·(-21)
Δ = 440.76
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{-(-5.4)-\sqrt{440.76}}{2*4.9}=\frac{5.4-\sqrt{440.76}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-5.4)+\sqrt{440.76}}{2*4.9}=\frac{5.4+\sqrt{440.76}}{9.8}
$