A classical mechanics problem by Ken Osako

Classical Mechanics Level 2

When the three blocks in the figure are released from rest, they accelerate with a magnitude of $1 \text{ m/s}^2$ . Block 1 has mass $M$ , block 2 has $2M$ , and block 3 has $2M$ . What is the coefficient of kinetic friction between block 2 and the table?

Take $g=9.8 \text{ m/s} ^2$ .