In the traditional method you write the bigger number above
the smaller number, lining up digits with the same place
values. Then subtract the digits in each column, starting from
the right.
Adjust 757 to 768 because 768 – 668 is easier to subtract.
To do this you need to add 11.
Now do the subtraction: 768 – 668 = 100
Amend this answer by subtracting 11.
Answer: 100 – 11 = 89