When the three blocks in Fig. $6-29$ are released from rest, they accelerate with a magnitude of 0.500 $\mathrm{m} / \mathrm{s}^{2} .$ Block 1 has mass $M,$ block 2 has $2 M,$ and block 3 has 2$M$ .
What is the coefficient of kinetic friction between block 2 and the table?