Object A moves at a steady 12 m/s. Object B starts from rest, gains 3 m/s every second, and keeps…

Step 1. At $t = 0$. $v_{BA} = 0 - 12\ \text{m/s} = -12\ \text{m/s}$: A pulls away, so B seems to go backward.

Step 2. At $t = 2\ \text{s}$. $v_B = 6\ \text{m/s}$, so $v_{BA} = 6\ \text{m/s} - 12\ \text{m/s} = -6\ \text{m/s}$.

Step 3. At $t = 6\ \text{s}$. Both move at $12\ \text{m/s}$: $v_{BA} = 0$, and the gap stays fixed.