A + 8 μC charge lies 4 m to the left of a - 10 μC charge. A - 12 μC charge lies 8 m to its right. What is the resultant force on the center charge?

both side charges will exert force on central charge towards left so the two forces add up. Thus resultant force on centre charge is

F = 9*10^9 x 8*10^-6*10*10^-6/16

+

9 * 10^9 * 10*10^-6*12*10^-6/64

= 9 * 10^9 * 10^ - 12/16 (80+30)

=6.18 * 10^-2 N

or

= 0.06 N