From Förberedande kurs i matematik 1

In this right-angled triangle, the side of length 17 is the hypotenuse (it is the side which is opposite the right angle). The Pythagorean theorem then gives

\displaystyle 17^2 = 8^2 + x^2

or

\displaystyle x^2 = 17^2 - 8^2\,\textrm{.}

We get

\displaystyle \begin{align} x &= \sqrt{17^2-8^2} = \sqrt{289-64} = \sqrt{225}\\[5pt] &= \sqrt{9\cdot 25} = \sqrt{3^2\cdot 5^2} = 3\cdot 5 = 15\,\textrm{.} \end{align}