Answer
$x$, $x+6$
Work Step by Step
For a given function $f(x)=x^2-5x+4$, to evaluate its value at $x=u$, we can replace $x$ with $u$, where $u$ can be an expression such as $u=x+6$ to get $f(u)=u^2-5u+4$ or $f(x+6)=(x+6)^2-5(x+6)+4$. Thus the answers to this question are $x$ and $x+6$.
