Answer:
Placing a ladder up against a building will create a right triangle, with the legs being the ground and the building and the hypotenuse being the ladder. Since we have a right triangle and we know two sides, we can use the Pythagorean Theorem to solve for the 3rd:
62+b2=102
36+b2=100
b2=64
b=8
So the ladder will reach 8 feet up.
Hope this helps!