Answer
$M=32 \frac{abc}{d}$
Work Step by Step
Since $M$ is jointly/directly proportional to $a$, $b$, and $c$, and inversely proportional to $d$, we have:
$M=k \frac{abc}{d}$
We are given: $M=128$ and $a=d$ and $b=c=2$, so we have:
$128=k \frac{a*2*2}{a}=4k$
$k=128/4=32$
So the equation is:
$M=32 \frac{abc}{d}$