As shown in the figure, a block of mass $M_{1}=0.250$ $\mathrm{kg}$ is initially at rest on a slab of mass $M_{2}=0.420 \mathrm{~kg}$, and the slab is initially at rest on a level table. A string of negligible mass is connected to the slab, runs over a frictionless pulley on the edge of the table, and is attached to a hanging mass $M_{3}=1.80 \mathrm{~kg} .$ The block rests on the slab but is not tied to the string, so friction provides the only horizontal force on the block. The slab has a coefficient of kinetic friction $\mu_{k}=$ 0.340 with both the table and the block. When released, $M$, pulls on the string, which accelerates the slab so quickly that the block starts to slide on the slab, Before the block slides off the top of the slab:
a) Find the magnitude of the acceleration of the block.
b) Find the magnitude of the acceleration of the slab.