Answer :
The force for the case 1 is 64 pounds and the distance is 9 feet, it is:
[tex]\begin{gathered} \frac{k}{9^2}=64 \\ k=5184 \end{gathered}[/tex]Now, if the force is 36 pounds, let solve for the distance:
[tex]\begin{gathered} \frac{k}{r^2}=36 \\ \frac{5184}{r^2}=36 \\ r^2=144 \\ r=12 \end{gathered}[/tex]Hence the answer is 12 feet.