c = 2x + 1

b = 15

x = ?

(2x + 1)^2 = 15 ^2 + x^2

(2x + 1)(2x + 1) = 225 + x^2

4x^2 + 4x + 1 = 225 + x^2

4x^2-x^2 + 4x + 1 - 225 = 225 - 225 + x^2 - x^2

3x^2 + 4x - 224 = 0

(x-8)*(3x + 28) = 0

x-8 = 0

x = 8

2x + 1 = 16 + 1 = 17

here we go.

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Pythagorean theorem