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You'll have to go to some exhaust shop. They will have to do some welding and replacing of completely rusted sections. When mine broke in a similar location I was charged $75 to have it fixed.
1.) Buy a new muffler resonator.
2.) Go to local junk yard and pull one
3.) Exhaust shop
My vote is exhaust shop as it is the fastest and most cost effective, IMO. Cheapest is probably pulling one off a junked car but it might be rusted plus the time to do all that. Like Brotato said I don't see it costing more then $75.
To be clear, your muffler looks fine, the part that broke was off the resonator which is legally allowed to be replaced with just straight pipe but you will definitely notice a annoying sound without it.
Just whatever section is broken/rusted. If its only a foot then just replace that.
I was looking at your muffler and thinking muffler but like Slump said its the resonator flange that is broken. Depending on how much is left you could weld a new flange and extension onto it. Or remove it and weld in a straight piece (might sound like crap though).