first, the negative charge has nothing to do with the E field at that postion.
You can add (as vectors) the three contributions from the three charges.
b) force= Eyoufoundabove*qfromnegativecharge
Three equal positive point charges of magnitude Q = 8.00ì C are located at three corners of a square of edge length d = 11.8 cm. A negative charge -24.00ì C is placed on the fourth corner. At the position of the negative charge, what is the magnitude of the electric field due to the three positive charges?
2.What is the magnitude of the attractive force on the negative charge?
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