Find the missing length. Round to the nearest tenth, If necessary.

Using the formula a² + b² = c² but reversing it to c² - b² = x² where x replaces the a for comprehension reasons

so this is the formula
c² - b² = x²

insert the numbers on the triangle in the picture
21² - 8² = 377

since 377 is squared and not the side length, we need to square root it to find x
√377 = 19.4~

x = 19.4

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