The figure below shows three small, positively charged spheres at three corners of a rectangle. The particle at the upper left has a charge of 6.00 nC, the one at the lower left has a charge of 3.00 nC, and the one at the lower right has a charge of 7.00 nC. The rectangle's horizontal side has a length of 5.50 cm and its vertical side has a length of 2.00 cm.
What is the electric potential (in V) at the upper-right corner of the rectangle? (Assume the zero of electric potential is at infinity)
What would be the potential (in V) at the upper-right corner if the charge at the lower left was -3.00 nC instead?