Answer: ![12\ inches](https://tex.z-dn.net/?f=12%5C%20inches)
Step-by-step explanation:
Let be "x" the height inches (in the model) of a 84 foot building.
According to the information provided in the exercise, we know that the height of a two-story building is 21 feet and in the scale model its height is 3 inches.
Knowing this, we can set up the following proportion:
![\frac{3}{21}=\frac{x}{84}](https://tex.z-dn.net/?f=%5Cfrac%7B3%7D%7B21%7D%3D%5Cfrac%7Bx%7D%7B84%7D)
The final step is to solve for "x" in order to calculate its value. We get that this is:
![\frac{3}{21}=\frac{x}{84}\\\\(84)(\frac{3}{21})=x\\\\x=12](https://tex.z-dn.net/?f=%5Cfrac%7B3%7D%7B21%7D%3D%5Cfrac%7Bx%7D%7B84%7D%5C%5C%5C%5C%2884%29%28%5Cfrac%7B3%7D%7B21%7D%29%3Dx%5C%5C%5C%5Cx%3D12)
Therefore, an 84 foot building will be 12 inches tall in the model.