Answer
The magnitude of the net gravitational force on the central particle is $~~\frac{3GM^2}{d^2}$
This force is directed toward the left.
Work Step by Step
Note that all of the gravitational forces on the central particle cancel out except for the gravitational force from the particle of mass $3M$ on the left side of the square.
We can find the magnitude of the net gravitational force on the central particle:
$F = G~\frac{(3M)(M)}{d^2}$
$F = \frac{3GM^2}{d^2}$
Note that this force is directed toward the left.
