Solution to task 308 from Chertov´s for extramural stud

Solution to task 308 from Chertov´s for extramural stud

308. At the vertices of the square there are identical charges Q1 = Q2 = Q3 = Q4 = 8.10-10 Cl. What negative charge Q must be placed in the center of the square in order for the force of mutual repulsion of positive charges to be balanced by the force of attraction of the negative charge?