Answer
The error is in the first step.
The correct solution is $f(5)=13$.
Work Step by Step
We have $x=5$.
Use the second equation.
$f(x)=x+8,$ if $x \geq5$
Substitute $5$ for $x$.
$\Rightarrow f(5)=5+8$
Simplify.
$\Rightarrow f(5)=13$
Hence, the error is in the first step.
The correct solution is $f(5)=13$.
