Solve the all given part of following question:
Question: Three equal positive point charges of magnitude Q = 3.00 µC are located at three corners of a square of edge length d = 7.9 cm. A negative charge -9.00 µC is placed on the fourth corner.
Part I: At the position of the negative charge, what is the magnitude of the electric field due to the three positive charges?
Part II: What is the magnitude of the attractive force on the negative charge?
Please provide any help is greatly appreciated!