2 blocks are accelerated across a horizontal frictionless surface as shown in a diagram (block on block getting pulled by a force, larger block on bottom). TH friction forces keep the 2 blocks from sliding relatice to each other, and the two move witha same acceleration. If F= 13.00 N, m1= 3.500 kg, and m2 = 6.200 kg, what is the size of the FRICTION FORCE of the large block on the small block? I have no idea where to start! Please help!