Let the positions in your current list be 1,2,3,4. So sqrt(21)=1, -cbrt(27)=2, -sqrt(16)=3, and 4th root (25)=4. These are just position placeholders, they aren't actually equal. Firstly, we find the ones that are integers. The negative of the cube root of 27 is -3 and the negative of the square root of 16 is -4. The fourth root of 25 is clearly smaller than the square root of 21 but it is nonnegative, so the list goes 3,2,4,1.