You say a plastic line? Take off the brass nut on one end. I'm pretty sure you will find a brass ferrule under the nut. Remove a piece of the hose and go to your local farm or automotive supply store. Buy a replacement hose long enough to replace the hose in question and two brass ferrules the same size as the hose. When you replace the hose slip the brass nut over the hose, then the ferrule. Firmly push the new hose into the fitting where the old hose came out of and tighten the brass nut. The ferrule will tighten onto the hose when you tighten the nut on each end. If that seems like too much work then go to the same store and buy just a repair coupling the same size as the plastic hose. Cut the hose square on each end and assemble the repair coupling over the hose. Repairing the hose is only a good idea if the hose isn't very brittle. Just bend the hose. If it snaps easily just replace it.