Answer:
13.75 foot
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Use Pythagoras theorem where hypotenuse is 15 feet and base is 6 feet.
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Let the length of building be h.
a² + b² = c²
h² + 6² = 15²
h² + 36 = 225
h² = 225 - 36
h² = 189
h = √189
h = 3√21
h ≈ 13.75 foot
