The building, the ladder, and the ground form a right triangle as shown below:
The ladder is the hypotenuse of the triangle and the height H and the horizontal distance (3 ft) are the legs.
Applying the Pythagora's Theorem:

Solving for H:
![\begin{gathered} H^2=11^2-3^2 \\ H=√(11^2-3^2) \\ H=\sqrt[]{121-9} \\ H=\sqrt[]{112} \\ H\approx10.58 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/cu3dvxj1fd196lyuzpnymlrm4nax1s5vvq.png)
The building is 10.58 feet tall