Answer:
about 17.29 feet
Step-by-step explanation:
Draw a diagram of the problem, it'll help you "see" the solution easier.
Basically, you have a right-triangle so you can apply Pythagorean theorem:
Let h = height the ladder reaches
h^2 + 5^2 = 18^2
h^2 = 18^2 - 5^2
h^2 = 324 - 25
h^2 = 299
h = sqrt(299)
h = 17.29 feet
