class number of $\mathbb{Q}(\sqrt{17})$

Question

I have to show that the class number of $K=\mathbb{Q}(\sqrt{17}) $ is $1$ .

First I know that $\sqrt{17}/2<3$ . Thus I only need to consider primes with norm $2$ . My problem is to show that these primes $a=(2,\sqrt{17}-1) ,b=(2,\sqrt{17}+1)$ are principal.

Thanks for the help .