hi i really need help

(-1/3)(1/6)square((-1/3)(1/6)-(-1/3)square))=

1/108(-1/18-1/9)=1/108(-1/9)=1/972

i know i got the wrong answer but i tried to find out several ways but i get the wrong answer " the answer suposed to be 1/648" how \HELP :confused: