My 400mm lens is so hard to slide out when I go to zoom what do I do

Turn the 'lock/friction ring' (metal band just below the black rubber focusing ring) counter clockwise whilst holding the focusing 'ring' still. The friction/lock ring is used to prevent 'lens creep' when carrying the camera with lens pointing down, or using the lens when it is pointing down. Juust turn it until it applies the level of friction you prefer for operating the lens. If you have already tried this and it won't budge it may be the bearings are shot and you will need to contact a canon service centre for repair.