Hints. Note that under the given conditions the tension in the string is constant all along the string. The tension is equal to the magnitude of each of the forces exerted by the string on the two blocks.

Make free-body diagrams for the two blocks, and set up your axes. Then write down the Newton's-Second-Law equations as applied to the two blocks. You only need to consider the components in the directions of motion of the two blocks.

You should obtain two equations for the tension and the magnitude of the acceleration of the blocks. Solve these equations for the tension.