Given data:
The given expression for the Hooke's law is,
[tex]\begin{gathered} y\propto x \\ y=kx \end{gathered}[/tex]Substitute 15 pounds for x and 9 inches for y in the above expression.
[tex]\begin{gathered} 9=15x \\ x=0.6 \end{gathered}[/tex]The expression for the Hooke's law is,
y=0.6x
Substitute 61 pounds for x in the above expression.
y=0.6(61) in
=36.6 in
Thus, the final exttension in the spring is 36.6 in.