A car waits at a red light. When it turns green, the car speeds up steadily from rest and reaches…

Step 1. Acceleration. $a = \dfrac{21\ \text{m/s}}{7.5\ \text{s}} = 2.8\ \text{m/s}^2$.

Step 2. Distance. Average velocity $\dfrac{0\ \text{m/s} + 21\ \text{m/s}}{2} = 10.5\ \text{m/s}$, so $s = 10.5\ \text{m/s} \times 7.5\ \text{s} = 78.75\ \text{m}$.