Trek 750 multitrack chainnsize and length? It has 3 gears in the front and 7 gears in the rear. And the derailers are suntour XC-Master.
Chains are highly standardized, but are specific to the number of sprockets in the rear cluster (because the spacing of those sprockets is narrower the more of them there are).

You simply need a chain marked as '6/7/8 speed' ('speed' here refers to number of sprockets in the rear cluster). 

All chains are sold with more links than are needed. They are sized to each bicycle, cut to the proper length and joined. 

Instructions for sizing, cutting and joining the chain are easy to find, but here's my favorite.

Buying a chain with a 'quick-link' type joining link is preferable as these are easier to join that those that use a joining pin that must be driven in with a chain tool.