Given, mass of the block = M
Coefficient of friction between the block
and the wall = $\mu$
Let a force F be applied on the block to hold the block against the wall. The normal reaction of mass be N and force of friction acting upward be f.
In equilibrium, vertical and horizontal forces should be balanced separately.
But force of friction (f) =$\mu$ N
= $\mu$F [using Eq. (ii)] ...(iii)
From Eqs. (i) and (iii), we get
$\mu$F = Mg
or F = Mg/$\mu$F