Question: NLPSolve..first time user, simple constraint problem

I am a transplant from to Mathematica to Maple and still getting used to things. Could somebody please help me finding the maximum of f with repect to the constraints and with the variables below. I keep getting  syntax error, no matter how many times I check the documentation page. Thank you very much

f:=1/((4 a - 3 M) (-4 b + 3 M)) (-9 M^3 - 16 a b x +
    12 a M x + 12 M^2 x + 16 b x^2 - 12 M x^2 + 12 b M y +
    12 M^2 y - 16 b x y - 16 M x y - 12 M y^2 + 16 x y^2)

constraints {0 < a < x < M, 0 < b < y < M}

variables={x,y}

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