A stone is dropped from a bridge 78.4 m above the water…

Step 1. Time. $t = \sqrt{\dfrac{2 \times 78.4\ \text{m}}{9.8\ \text{m/s}^2}} = \sqrt{16\ \text{s}^2} = 4\ \text{s}$.

Step 2. Speed. $9.8\ \text{m/s}^2 \times 4\ \text{s} = 39.2\ \text{m/s}$.