The Solution:
Given:
[tex]\begin{gathered} L=length=? \\ \\ W=width=16in \\ \\ A=area=21ft^2 \end{gathered}[/tex]Step 1:
Convert 21 square feet to square inches.
Recall:
[tex]\begin{gathered} 1\text{ foot}=12\text{ inches.} \\ So, \\ 21ft^2=21\times12=252\text{ }inches^2 \end{gathered}[/tex]Step 2:
Apply the formula for area.
[tex]\begin{gathered} A=L\times W \\ \\ 252=L\times16 \end{gathered}[/tex]Divide both sides by 16.
[tex]L=\frac{252}{16}=\frac{63}{4}=15\frac{3}{4}\text{ }inches[/tex]Therefore, the correct answer is:
[tex]15\frac{3}{4}\text{ inches}[/tex]