A cyclist at 10 m/s brakes, and her speed falls at a steady 2.5 m/s²…

Step 1. (a) $0 = 10\ \text{m/s} - 2.5\ \text{m/s}^2 \times t$, so $t = 4\ \text{s}$.

Step 2. (b) $s = 10\ \text{m/s} \times 4\ \text{s} - \tfrac12 \times 2.5\ \text{m/s}^2 \times (4\ \text{s})^2 = 40\ \text{m} - 20\ \text{m} = 20\ \text{m}$.

Step 3. (c) $0 = 100\ \text{m}^2/\text{s}^2 - 2 \times 2.5\ \text{m/s}^2 \times s$, so $s = 20\ \text{m}$. They agree.