A Magic Difference Square

Place the numbers 1 through 9 into a 3x3 grid, such that in each of the 8 directions of 3 squares (horizontally, vertically and diagonally) the sum of the first and last numbers minus the center number gives the same result. How many different answers exist?

Source: Original, based on a puzzle in Michael Holt's Math Puzzles and Games, Vol. 1, Page 43.

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