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12x - 13
36xy / 3y = 12x
39y / 3y = 13
Deadshot's answer makes an assuming that you intended (36xy-39y)/3y With inline math statements it is critical to use ( ) to make things clear otherwise precedence of operations will give you something different than you expected in this case division before subtraction
Another issue is that you didn't indicate what to do with the expression. What was the command? Simplify? that again was the assumption.
So if you were indeed being asked to simplify then the answer to the expression as you typed it would be 36xy-13.
I suppose that's what frustrates math students that math can be very precise. Implying things many times leads to errors.