Weed eater 20 4.5 lawn mower used 3 times now it won't start unless i prime it then it dies unless i keep priming it what is wrong?

The gas is most likely getting old. The easy way to fix it hopefully is to add some dry gas to it. Take a carb cleaner and spray it into the air intake while it's starting to run, that should help burn through any gunk too. If that doesn't work, take the gas out and replace with new gas and add stabilizer to it. If that still doesn't work, you may have to take the carbs out and clean out the innards. That is not a fun one if you haven't done it before.