Answer:
One should draw a diagram showing the placement of the charges.
When both charges of -Q are placed there will be no force in the vertical direction because of cancellation of forces.
A line from one corner of the square to the other makes an angle of 45 degrees,
2 cos 45 deg will be the force due to one charge.
2 * .707 = 1.41 N
So for both charges the force is (horizontal, vertical is zero)
2 * 1.41 = 2.82 N
b) is correct
True........don't blame me if i'm wrong.<span />
Answer:
I have attached an easy to understand solution to your question. I just have to use a paper close to me at the moment