The block m is on block M. An external force apply to M.
If we think m=3Kg and M=6Kg ,between M and floor there is no friction and between m and M μ=0.4.
What is the maximum force with which the lower block can be pulled horizontally so that the two blocks move together without slipping?

refer to the attached diagram ...
$f \le \mu \cdot mg$
proceeding by treating it as an unknown value ...
$\color{blue}{f} = ma_1 \implies a_1 = \dfrac{\color{blue}{f}}{m}$
$F\color{red}{f} = Ma_2 \implies \dfrac{F\color{red}{f}}{M} = a_2$
no slippage, so $a_2 = a_1$
$\dfrac{F\color{red}{f}}{M} = \dfrac{\color{blue}{f}}{m}$
Newton's 3rd Law says $\color{blue}{f} = \color{red}{f}$ ...
solving for $f$
$f = \dfrac{mF}{M+m} < \mu \cdot mg \implies F < \mu g(M+m) = 36 \, N$