On A Weed Eater XT300, there are 2 lines to the gas tank, one is larger than the other, my question is, " in the gas tank which line does the fuel filter attach to."

The one that comes from the longer connection on the primer bulb.

Tank guy? The one that comes from the primer bulbs longest connection is the one to the filter. This has nothing to do with the lines being rotten. All you have to do is look at the back side of the primer bulb.

Thanks

Did you get your problem solved?