My guess is that these two lengths are the short sides of a right-angled triangle, so the length of the longest side is sqrt(17^2+8^2)=sqrt(289+64)=sqrt(353)=18.79 feet roughly. If 17 feet is the length of the longest side then the missing length is sqrt(289-64)=sqrt(225)=15 feet exactly. This looks like the answer you need!