A ladder is 12 metres and leans against a tall wall. The foot of the ladder is 9 metres from the base of the wall. How far up the wall will the ladder reach?
You're looking at a right-angle triangle. Use the Pythagorean Theorem.
a^2 + b^2 = c^2
a^2 + 9^2 = 12^2
a^2 + 81 = 144
a^2 = 63
a = 7.94 meters