A 30 kg trolley moves at 3 m/s. The only force along its motion is friction, a steady 18 N.

Step 1. (a) $a = -\dfrac{18\ \text{N}}{30\ \text{kg}} = -0.6\ \text{m/s}^2$.

Step 2. (b) $0 = 3\ \text{m/s} - 0.6\ \text{m/s}^2 \times t$, so $t = 5\ \text{s}$.

Step 3. (c) $s = \tfrac12 \times (3\ \text{m/s} + 0\ \text{m/s}) \times 5\ \text{s} = 7.5\ \text{m}$.