1. The problem statement, all variables and given/known data Find the electric field and the potential at the center of a square 2.0 cm with charges of +9.0uC at one corner of the square and with charger of -3.0uC at the remaining 3 corners. 2. Relevant equations the sum of Vi= the sum of KQi/ri 3. The attempt at a solution I saw that the equation was re-arranged this way but I'm not sure how they arrived there: E= Ea+Eb+Ec+Ed = Ea+Eb+Ec-Eb= Ea+Ec= (K|qa|/r^2 + K|qc|/r^2) towards C when you plug in the #'s the answer is 5.4x10^8 N/C toward c. what I dont understand is how they concluded that that was the formula to use.