According to Shimano specs that derailleur is 42T compatible.
https://bike.shimano.com/en-EU/produ...-M6000-GS.html
Edit: upon closer inspection it looks that what you have is in fact the SGS version which is rated only for 36T. Then yes, you need a derailleur extension for it to work with 42t casette.
https://bike.shimano.com/en-EU/produ...M6000-SGS.html
Last edited by subgrade; 09-13-19 at 08:26 AM.