Answer: 20 feet
Explanation:
This represents a right angled triangle with the length of the slide being the hypotenuse, the height of the ladder being the height of the triangle and the distance from the base of the ladder being the length.
Solve with Pythagoras rule:
c² = a² + b²
c = hypotenuse
a= length
b = height
29² = 21² + b²
841 = 441 + b²
b² = 841 - 441
b² = 400
b = √400
b = 20 feet