
Using Coulomb's law the electrostatic force on A due to C
FAC=(rAC)2KqAqC=9×10−4k×5×0.3×10−12=9×10−49×109×5×0.3×10−12
=1.5×10=15N
Similarly, the force on A due to B
FAB=(rAB)2KqAqB=9×10−49×109×5×0.16×10−12=8N
We can see that the forces are perpendicular to each other. Therefore, the net force experienced by charge at A is,
F=FAC2+FAB2=152+82=289=17N