Rear mech identification

need a new rear mech but dont know wether i need a medium or long cage.
mech is a SRAM X9 10 spd..

cage length depends on chainrings and cassette size, so without those details no definite answer is possible.

However for most set ups you can get away with a medium, but a long, looks crap, but works on even more setups.

Good explanation here.

Front is 42/27 Rear is 12-36 .

definately go with a long then. if you were running 36,24 you could get away with a medium. but with that gearing its not worth chancing a medium cage

Cheers 🙂 will order the long one .