Two blocks of masses $2\ \text{kg}$ and $3\ \text{kg}$ lie on a smooth horizontal floor, joined by a light spring. A constant horizontal force of $12\ \text{N}$ pulls the $3\ \text{kg}$ block away from the other one. At a certain instant, the acceleration of the $2\ \text{kg}$ block is $1.5\ \text{m/s}^2$. The acceleration of the $3\ \text{kg}$ block at that instant is