Chapter 2 - Section 2.7 - Combining Functions - 2.7 Exercises - Page 217: 45

$3$

Based on the table, the $y-$value of $f$ at $x=5$ is $6$. Hence, $f(5)=6.$ The $y-$value of $f$ when $x=6$ is $3$. Hence, $f(6)=3$. The given composition function, $ (f \circ f)(5) ,$ evaluates to \begin{array}{l}\require{cancel} f(f(5)) \\\\= f(6) \\\\= 3 .\end{array}