9.8=14.7t-4.9t^2

Solution for 9.8=14.7t-4.9t^2 equation:

9.8=14.7t-4.9t^2

We move all terms to the left:

9.8-(14.7t-4.9t^2)=0

We get rid of parentheses

4.9t^2-14.7t+9.8=0

a = 4.9; b = -14.7; c = +9.8;
Δ = b2-4ac
Δ = -14.72-4·4.9·9.8
Δ = 24.01
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{-(-14.7)-\sqrt{24.01}}{2*4.9}=\frac{14.7-\sqrt{24.01}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-14.7)+\sqrt{24.01}}{2*4.9}=\frac{14.7+\sqrt{24.01}}{9.8}
$