Figure 1 depicts a square of side length $s = 20.0$ mm on whose corners are arrayed four charges. Each charge has the same magnitude of $q = 1.00$ nC. Three of the charges are positive and one of them is negative, so that $q_1 = q_2 = q_4 = +q$ and $q_3 = -q$. Four points are labeled on the outer square; points $A$, $B$, $C$, and $D$. Each of these points lies at the center point of one side of the outer square.
What is the electric potential at point $C$?