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You know by the Pythagorean theorem, so all that's left is a little algebra. Since both sides are equal, their square roots are equal, so getting there just depends on knowing the theorem and a little bit of algebra. If it's the first part that's giving you trouble you should study the proof until you can see why it's true, because then remembering it is easy, but most just memorize it. If it's the second then review your algebra. Your textbook sucks, because all high school math textbooks suck, but it's not impossible to make sense of. 6² + 8² = c² c = √( 6² + 8²) = 10