First, to analyze this..you should illustrate the problem..
solve first for DM
since DE and DF is equal then DM must me cutting EF into two equal parts, EM=EF=8
since it would be a right triangle then we can use the Pythagorean theorem in whinc hyp^2=a^2+b^2
threfore substituting, 17^2=8^2+(EM)^2
i"m sorry i have no idea on finding EN..that was all i know