Question: How to define a function?

I have an equation of the plane Q: x + y + z - 1 = 0. Now i want to define a function 


and i calculate the value f(1, 2, 3), but Maple understand. Plese help me.

If i want to substitute the coordinates N(a + 1, b + 2, c + 3) into the equation of the plane Q. What must i do?

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