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Draw the FBD.
Get the eqs in x and y
You should end up with Fn+20sinθ = mg
and 20cosθ- mu Fn = ma
This gets messy quickly, but the general idea will be to maximize 20cosθ + 8sinθ. I get 21.541 as a max. Sticking this back into the originals gets an acceleration of .388.
This is a really complicated question, are you sure some piece of info wasn't left out?