45, 44, 36, 32, 31, 26, 22, 21, ?

I think you may have missed something:

(1) 44 - 36 = 8, not 6

(2) 31 - 26 = 5, not 6

(3) even if the subtraction had been correct, the next term in your sequence would be 21 - 6 = 15, not 21 - 4 = 17.

I think Vernon's solution was probably correct.