The Trek 7.1FX comes with a freewheel (no lock ring but internal spines), not a cassette. You need this tool http://www.amazon.com/Park-Tool-Free...freewheel+tool

With a freewheel tool you also don't use a chain whip. You just insert the tool, grab a big Crescent wrench (or other brand) and turn lefty loosey; I sometimes put the wheel against a wall (with the tire still in place) to stop the tire from turning). Also freewheels can really hold on and need more torque than just a Crescent wrench. If so let us know and well give you some more pointers.