Answer
$24$ inches long
Work Step by Step
You have the expression $21+0.5n$ which is the length in inches.
$n$ is the age of a baby in months.
Plug in $6$ for $n$ because you have to find the length of the baby after $6$ months:
$=21+0.5n
\\=21+0.5(6)
\\=21+3
\\=24$
After $6$ months a baby would be $24$ inches long.
