four charge +q are placed at `a`distance on the axes. what is net force on charge placed at coordinates (0,0,z)

I am considering the charge at(0,0,z) also as q. Now according to formula net force due to any one will be q²/4π€°(z²+a²). Now its along z axis component will be retained and perpendicular components will be cancelled out because force due to all 4 will be pair wise opposite. Hence net force =4×F×cosx where F is force due to one and x is the angle made with vertical.. putting the values we can get answer