Is this a function even though there’s a -1 and a 1 in the same x column?

yes, because they have a common difference and it makes sense. in order to determine if this is a function, you would have to find their difference for each column. the x column has a difference of 2 for all. similarily, the y column has a difference of 2 for all. therefore, it is a function.

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this is answer of 144 cubic meters