I'm having a lot of trouble trying to solve this problem:
Three identical point charges of magnitude $+1.0$ $mC$ are located at the corners of a square that is $1.0$ $m$ on a side. A point charge of magnitude $-1.5$ $mC$ is placed at the remaining corner. What is the electric field strength at a point halfway between 2 adjacent positive charges?
My thinking was that you can just find the strength of one charge to the center and just double it because they're identical charges. Why is this method wrong and how do I solve this problem?
The correct answer is $8.63\times 10^6$ $N/C$, but I have no idea how to get this.