When a point charge of (+q) is placed at corner (A) of a square (see figure), the electric potential at the center of the square...?

... is found to equal 7.00×10^-2 V. What is the electric potential at the center of the square when a charge (+q) is placed at corner (A), a charge (+q) is placed at corner (B), a charge (-q) is placed at corner (C), and a charge (+q) is placed at corner (D)?
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