It is an odd question, and I'm not completely sure I understand it.
If you simplify the polynomial, you just get 0, which would have a degree of 0.
If you are not supposed to simplify first, then the degree should be 5, as long as a and x are both variables. To get the degree of each term you add the exponents of all of the variables (so that the degree of 24x3y6 would be 9.)