A motorbike moving at 63 km/h brakes steadily and stops in 3.5 s…

Step 1. Acceleration. $a = \dfrac{0\ \text{m/s} - 17.5\ \text{m/s}}{3.5\ \text{s}} = -5.0\ \text{m/s}^2$.

Step 2. Distance. $s = \dfrac{17.5\ \text{m/s} + 0\ \text{m/s}}{2} \times 3.5\ \text{s} = 30.6\ \text{m}$.