Mechanics question help

I know this is physics but seeing as mechanics is maths at A-level, I thought some of you could help me ? It is part of a classical mechanics MOOC I am doing.



Thanks

Let the force by the wind on each block be $T$.
Let $F_1, F_2$ be the friction between the blocks and the friction between the lower block and the floor respectively.


The upper block is moving to the right at constant speed thus $F_1$ is acting towards the left and $F_1 = T$. Hence

$T = F_1 = \mu M_1 g$

$\Rightarrow M_1 = \frac{F_1}{\mu g}$


The lower block is moving to the left at constant speed thus $F_1$ on this block is acting towards the right and $F_2$ is also acting towards the right. Hence:

$F = F_1 + T + F_2 = 2F_1 +F_2$

$\Rightarrow F = 2F_1 + \mu (M_1 + M_2)g$

$\Rightarrow F_1 = \frac{F - \mu (M_1 + M_2)g}{2}$


Substitute this into the above expression of $M_1$, we have:

$M_1 = \frac{F - \mu (M_1 + M_2)g}{2 \mu g}$

Rearrange to make $M_1$ the subject, we get:

$M_1 = \frac{F}{3 \mu g} - \frac{M_2}{3}$