Question

Four charged particles are arranged at the corners (1, 2, 3 and 4) of a square with each side length L The particles at opposite corners have equal charge.
a). What will be the sign of charges at each corner to make net
electric force on each charge at corner 1 and 4 equal to 0.
b). Find the relationship between Q and q such that the electric
force on each particle with charge Q is zero.