Given three parallel conducting plates which are aligned perpendicular to the x-axis. They are labeled, from left to right as plate 1, 2 and 3 respectively. The corresponding plate charges are Q1 = -q, Q2 = 0 and Q3 = q. The width of the gap between 1 and 2 is d, and the width between plates 2 and 3 is d/2. Determine the magnitude of the potential difference across 1 and 3.