Trolley A has a mass of 3 kg and trolley B 9 kg.

Step 1. (a) A: $\dfrac{18\ \text{N}}{3\ \text{kg}} = 6\ \text{m/s}^2$. B: $\dfrac{18\ \text{N}}{9\ \text{kg}} = 2\ \text{m/s}^2$.

Step 2. (b) $\dfrac{6\ \text{m/s}^2}{2\ \text{m/s}^2} = 3\ \text{kg}$ times: B has three times the mass.

Step 3. (c) It doubles, to $12\ \text{m/s}^2$: acceleration grows in step with the force.