Four equal-magnitude (4.0 C) charges in vacuum are placed at the four corners of a square that

Four equal-magnitude (4.0 μC) charges in vacuum are placed at the four corners of a square that is 20 cm on each side. Find the electric field at the center of the square 

(a) If the charges are all positive, 

(b) If the charges alternate in sign around the perimeter of the square, 

(c) If the charges have the following sequence around the square: plus, plus, minus, minus.