Force is a vector, and it's critical to add forces as vectors whenever there is more than one force being exerted. Consider some two-dimensional situations, all involving the net force the positive charge at the center of a square experiences because of equal-magnitude charges placed at each corner of the square.

Configuration 1. The net force on the positive center charge is toward the top-right.
What are the signs of the equal-magnitude charges occupying each corner?
How many possible configurations can you come up with that will produce the desired force?

Configuration 2. The net force on the positive center charge is straight down.
What are the signs of the equal-magnitude charges occupying each corner?
How many possible configurations can you come up with that will produce the desired force?

Configuration 3. The net force on the positive center charge is zero.
What are the signs of the equal-magnitude charges occupying each corner?
How many possible configurations can you come up with that will produce the desired force?