Answer: 78.8 ft
Step-by-step explanation:
Similar triangles have proportional sides.
[tex]\dfrac{\text{building shadow}}{\text{building height}}=\dfrac{\text{flagpole shadow}}{\text{flagpole height}}\\\\\\\dfrac{35}{\text{building height}}=\dfrac{8}{18}\\\\\\\dfrac{35\times 18}{8}=\text{building height}\\\\\\\dfrac{35\times 9}{4}=\text{building height}\\\\\\\dfrac{315}{4}=\text{building height}\\\\\\\boxed{78.75=\text{building height}}[/tex]