we have the following:
[tex]\begin{gathered} y=k\cdot\sqrt{x} \\ 48=k\cdot\sqrt{64} \\ k=\frac{48}{8} \\ k=6 \\ y=6\cdot\sqrt{x} \end{gathered}[/tex]therefore, x = 712
[tex]\begin{gathered} y=6\cdot\sqrt{712}=6\cdot26.68 \\ y=160.1 \end{gathered}[/tex]therefore, the answer is 160.1