Work Shown
[tex]a^2 + b^2 = c^2\\\\8^2 + 15^2 = \text{x}^2\\\\\text{x} = \sqrt{8^2 + 15^2}\\\\\text{x} = \sqrt{289}\\\\\text{x} = 17\\\\[/tex]
For more information and examples, search out "Pythagorean Theorem".