Answer
$x$;
$6$
Work Step by Step
RECALL:
$f(a)$ represents the value of the function $f$ when $x=a$.
To find $f(6)$ in the given function, replace $x$ with $6$ to obtain:
$f(x) = 3x+5
\\f(6) = 3(6) + 5
\\f(6) = 18+5
\\f(6) = 23$
Thus, the missing expressions in the given statement are:
$x$;
$6$
