Walker, Halliday & Resnick — Electric Potential: Problem 42

42 Figure 24-42 shows a rectangular array of charged particles fixed in place, with distance $a = 35.0 \text{ cm}$ and the charges shown as integer multiples of $q_1 = 3.40 \text{ pC}$ and $q_2 = 6.00 \text{ pC}$. With $V = 0$ at infinity, what is the net electric potential at the rectangle’s center? (Hint: Thoughtful examination of the arrangement can reduce the calculation.)