To specify the next number, you would need to recognize some sort of pattern. The numbers are different squares and cubes; but they seem to be in some random order.
216
The next number in the sequence is... 64
125 (the sequence is successive numbers cubed)
64
Should there be a 16 in there somewhere? If so next three are 36, 49 and 64...
81
The next number will be 25 because 5 times 5 = 25
81
The next number in the sequence 2, 4, 16, 64 is 256.
The next number in the sequence is... 64
121
125 (the sequence is successive numbers cubed)
You are multiplying each number in the sequence by -2. Therefore, the next number in the sequence after 32 is -64.
729
49
144
64
64. It's the sequence f(n) = n^3