This is a method of subtraction that uses addition to subtract two multi-digit numbers. It is similar to regular subtraction.
It breaks the subtrahend into two parts, where part one is a nine/ten's complement, followed by a simple addition.
Note that Minuend - Subtrahend = Difference. Get the Minuend Complement and subtract the Subtrahend to solve for the Difference Complement. Add the Minuend to the Difference Complement. Remove the leading digit, '1'. This is the Method of Complements.

Enter:  Minuend   Subtrahend