We can use the Pythagorean theorem to solve this problem, which states that in a right triangle, the square of the hypotenuse (the longest side) is equal to the sum of the squares of the other two sides.
Let x be the length of the slide. Then we have a right triangle with legs of length 5 (the height of the ladder) and x, and hypotenuse of length 12 (the distance from the ladder to the bottom of the slide).
Using the Pythagorean theorem:
12^2 = 5^2 + x^2
144 = 25 + x^2
Subtracting 25 from both sides:
119 = x^2
Taking the square root of both sides:
x ≈ 10.91
Therefore, the slide needs to be about 10.91 feet long.