$24$ 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.