draw a rectangle triangle :
sin B=8/17 that means that de biggest side are 17 ("hipotenusa"), and another side is 8. Applying de theorem of pythagoras:
[tex]17^{2} =8^{2} +x^{2} \\x^{2} =17^{2} -8^{2}\\x=\sqrt{17^{2} -8^{2}} \\x=5\sqrt{5}[/tex]
