Force on -7 uC charge due to charge placed at x = - 10cm
[tex]F = \frac{kq_1q_2}{r^2}[/tex]
now we will have
[tex]F_1 = \frac{(8.9875 \times 10^9)(3\mu C)(7\mu C)}{0.12^2}[/tex]
[tex]F_1 = 13.1 N[/tex] towards left
similarly force due to -5 uC charge placed at x = 6 cm
[tex]F = \frac{kq_1q_2}{r^2}[/tex]
now we will have
[tex]F_2 = \frac{(8.9875 \times 10^9)(5\mu C)(7\mu C)}{0.04^2}[/tex]
[tex]F_2 = 196.6 N[/tex] towards left
Now net force on 7 uC charge is given as
[tex]F = F_1 + F_2[/tex]
[tex]F = 196.6 + 13.1 = 209.7 N[/tex]
towards left