In the figure, every charge on the perimeter of the square has an identical partner (the same magnitude and sign) directly accross (through the center) and thus the same distance away from the center.  The net force on the center charge from each pair is therefore zero.  There is only one unpaired charge on the perimeter, which is responsible for all of the net force.

The answer for (a) is therefore a single application of Coulombs force law, kQq/d^2.  (According to the description, the square has a side of length 2d.)

(b) Since the unpaired perimeter charge is opposite sign from the central charge, they attract each other.  So the force is leftward.