Newton's laws of motion — practice question

Particles $P$ and $Q$, whose masses are $7\,\text{kg}$ and $3\,\text{kg}$ respectively, are secured to the two ends of a light inextensible string. The string goes over two small smooth pulleys fixed at the two ends of a horizontal table. Both particles hang vertically beneath the pulleys. Initially, both particles are at rest, $0.5\,\text{m}$ below table level and $0.4\,\text{m}$ above the horizontal floor (see diagram).