your question is quite simple but it is a bit tough for us to answer at this platform because some of the limitations . however you can use the principle of superposition to answer this questiion ,and your question willbe solved easily.however it can be found on the internet as well
https://www.topperlearning.com/doubts-solutions/four-charges-each-equal-to-q-are-placed-at-the-four-corners-of-a-square-and-a-charge-q-is-placed-at-its-centre-of-the-square-if-the-system-is-in-equil-imypzrcc
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