Ap physics B
posted by christina .
Four positive charges are fixed at the corners of a square, as shown above. Three of the charges have magnitude Q, and the fourth charge has a magnitude 2Q. Point P is at the center of the square at a distance r from each charge.
22. What is the electric potential at point P ? (A) Zero
(B) kQ r
(C) 2kQ r
(D) 4kQ r
(E) 5kQ

It is so simple: I suspect you are over thinking.
Potential is a scalar, not vector. So add the four potentials, three are the same, and one is double, (2+3)
V=5k(q/r)