Answer
$$M=32\frac{abc}{d}$$
Work Step by Step
$M$ is jointly proportional to $a,b$ and $c$ and inversely to $d$ is represented as:
$M=k\frac{abc}{d}$
To write it in a general form, we have to find $k$ constant.
$M=128$; $a=d$; $ b=c=2$
$128=k\times4$
$k=32$
$M=32\frac{abc}{d}$
