Answer:
3. µs g /7
Explanation:
The largest Force appear when the maximal friction Force is required.
Second Newton law for the small block:
[tex]F_friction=u_s*N=u_s*(mg)[/tex]
[tex]F-F_friction=ma[/tex]
[tex]F-u_s*(mg)=ma[/tex]
Second Newton law for the Big Block:
[tex]F_friction=7ma[/tex]
[tex]u_s*(mg)=7ma[/tex]
[tex]a=u_s*g/7[/tex]