The correct method is-
1. write the positive number of a given problem
2. take 1's complement of a positive number
3. take 2's complement of step 2 & get the result.
the solution of above problem is-
57=00111001(represent in binary)
1's compliment form=11000110
2's compliment form= 11000110+1=11000111 is correct representation of given problem.
see link: Gate-cse-2002