Looking at the only columns where we have both x and f(x), we can see this couples:
[tex]\begin{array}{c|c}x&f(x)\\1&1\\49&7\\25&5\end{array}[/tex]
We can see that the output is always the square root of the input, so the function is
[tex]f(x)=\sqrt{x}[/tex]
and the table completes to
[tex]\begin{array}{c|c}x&f(x)\\9&3\\1&1\\100&10\\4&2\\49&7\\16&4\\0&0\\25&5\\20&\sqrt{20}\end{array}[/tex]
