#"force between "Q_A " and "Q_B" is":#
#color(blue)(F_1)=k((-7)*(-2))/7^2#
#color(blue)(F_1)=(14k)/49#
#"force between "Q_B " and "Q_C" is :"#
#color(red)(F_2)=k*((-2)*(-2))/9^2#
#color(red)(F_2)=(4k)/81#
#color(blue)(F_1)" and "color(red)(F_3) " are opposite directions"#
#color(green)(F_"net")=color(blue)(F_1)-color(red)(F_2)#
#color(green)(F_"net")=(14k)/49-(4k)/81#
#color(green)(F_"net")=(1134k-196k)/3969#
#color(green)(F_"net")=(938k)/3969#
#color(green)(F_"net")=(938*9*10^9)/3969#
#color(green)(F_"net")=2.13*10^9" "N#