Go here to get the manual for the mower
Murray customer support manuals
You'll need the serial number from the mower to do this. Damn If I can figure out why. Sears is the same way. Anyway, you should have a couple of linkages that suspend the mower deck from the tractor frame. These should have some sort of adjustment built in. You need to level it over pavement so that you can measure the deck height on both sides. You'll need the manual for more specific info.