From Förberedande kurs i matematik 1

In this right-angled triangle, the side of length \displaystyle \text{17} is the hypotenuse (it is the side which is opposite the right angle). Pythagoras' theorem then gives

\displaystyle \text{17}^{2}=8^{2}+x^{2}

or

\displaystyle x^{2}=\text{17}^{2}-8^{2}

We get

\displaystyle \begin{align} & x=\sqrt{\text{17}^{2}-8^{2}}=\sqrt{289-64}=\sqrt{225} \\ & =\sqrt{9\centerdot 25}=\sqrt{3^{2}\centerdot 5^{2}}=3\centerdot 5=15. \\ \end{align}