Here are the steps to show that the given 3x3 array will always form a magic square for any values of a, b, c:
1) Each row sums to 3a.
2) Each column sums to a+b+c, a, and a-b+c.
3) The diagonals sum to a+b+c and a-b+c, which are equal.
Since each row, column and diagonal sums to the same value (3a for rows, a+b+c for columns/diagonals), the 3x3 array will always form a magic square for any integers a, b, c.