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~
Answer: x = 19.4
