Question: compute a double integral (involves parameters) in Maple

I want to calculate above integral in Maple15,  the parameters n_1, n_2 and n_3 are all positive integers.
I tried to input the following command: int(int((1-x)*(1-y)(x+y)*(x^n_1)*(y^n_2)*(1-x-y)^n_3, y=0..1-x),x=0..1);

After I pressed enter it then appeared an error message: "Error, unable to match delimiters"


I'm new to Maple, could any one tell the correct command? I think I can learn the commands for computing a range of integrals like this once I know the correct command for this integral

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