Please see attached
The Attempt at a Solution
(a) The blocks can have the same acceleration due to static friction.
consider the 2 blocks on the left: Ff=fmg
then consider the 2 blocks on the right: Ff is also fmg? I am rather confused at this point... how does M come into the equation? because the answer for part (a) is 2(M+m)/(M+2m)f. There is no g anywhere in the equation, though from the equation, friction is to the normal, which is mg?