Change 72 km/h into m/s. (b) Change 12 m/s into km/h.

Step 1. (a) $72\ \text{km/h} \times \tfrac{5}{18} = 20\ \text{m/s}$.

Step 2. (b) $12\ \text{m/s} \times \tfrac{18}{5} = 43.2\ \text{km/h}$.

Step 3. (c) $90\ \text{km/h} = 25\ \text{m/s}$, so $25\ \text{m/s} \times 12\ \text{s} = 300\ \text{m}$.