Subtracting a two's complement value from another two's complement value
Converting 6AD3 to binary gives 0110 1010 1101 0011 (the division to 4's is just to make it readable)

Converting AF20 to binary gives 1010 1111 0010 0000
To complement the latter in two's complement, we take the bitwise negation (one's complement) and add 1.

So the one's complement is 0101 0000 1101 1111, and adding 1 gives us
0101 0000 1110 0000
Adding this to 0110 1010 1101 0011 we have:

0101 0000 1110 0000

+

0110 1010 1101 0011

=

1011 1011 1011 0011

Which is BBB3.