Since both drawings are similar and have a scale factor, we can say that all sides keep the same scamle factor, if the scale drawing is in a proportion of 3:1 means that all of its sides is 3 times the real objects sides.
write this as equations
[tex]\begin{gathered} 3\cdot a=21in \\ 3\cdot b=18in \end{gathered}[/tex]
to find the respetive values for a and b we divide the sides by 3
[tex]\begin{gathered} a=\frac{21in}{3}=7in \\ b=\frac{18in}{3}=6in \end{gathered}[/tex]
The correct answer is D.
