What would the force be if the separation between the two charges was adjusted to 7.31 ✕ 10-11m? (Physics)?

https://puu.sh/CFGak.png

I attempted to use the Coulomb's law above.

k_e = 8.99 *10^9 N* m^2/C2
r^2 = 7.31 *10^-11 m
but im not sure if I need Q1 or Q2. Or maybe I am approaching this problem the wrong way.
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