Let x be the length of the third side.
Thus:
[tex]\mathsf{25^2=17^2+x^2}\\\\\mathsf{625=289+x^2}\\\\\mathsf{x^2=625-289}\\\\\mathsf{x^2=336}\\\\\mathsf{x=\sqrt{336}}\\\\\boxed{\mathsf{x=4\sqrt{21}}}[/tex]
