Answer:
20.62 feet
Explanation:
The ladder propped against a building forms a right angle triangle
Height of the ladder (hypotenuse) = 21 feet
Base of the ladder from the building (adjacent) = 4 feet
Height of the building (opposite) = x feet
Hypotenuse ² = opposite ² + adjacent ²
21² = x² + 4²
21² - 4² = x²
441 - 16 = x²
425 = x²
x = √425
= 20.62 feet