If the sum of the 2 M-1 power of (M-3) x ^ 2Y and - 2 / 3x ^ 2Y is still a monomial, then the sum is ()

If the sum of the 2 M-1 power of (M-3) x ^ 2Y and - 2 / 3x ^ 2Y is still a monomial, then the sum is ()

The result of this formula is 6