Two electric charges are separated by a certain distance. The first charge is + 5 C and the second charge is + 1 5 C

 Two electric charges are separated by a certain distance. The first charge is $+5$μC and the second charge is $+15$ μC$.$ The magnitude of the electrical force the first charge exerts on the second charge is $6\mathrm{.}0$ mN$.$ How large is a magnitude of the electrical force for the second charge exerts on the first charge? $($In mN$)$