A body starts with velocity u and speeds up at a steady acceleration a…

Step 1. Rectangle. Area $ut$: the distance at the starting velocity.

Step 2. Triangle. Width $t$, height $v - u = at$: area $\tfrac12 \times t \times at = \tfrac12 at^2$.

Step 3. Add. $s = ut + \tfrac12 at^2$.

Step 4. (b) $3\ \text{m/s} \times 5\ \text{s} + \tfrac12 \times 2\ \text{m/s}^2 \times (5\ \text{s})^2 = 15\ \text{m} + 25\ \text{m} = 40\ \text{m}$.