64h + 30h + 36h + 30h + 31h + 32h + 33h + 34h + 35h = F9h

In Hex, A = 10, B = 11, C = 12, D = 13, E = 14, F = 15 and the base is 16

Now, consider the addition..

4 + 0 + 6 + 0 + 1 + 2 + 3 + 4 + 5 = 25

If you are adding in Decimal, Int(25/10) = 2 will be carry and remainder 5 will be the sum at the unit place.

But in hex, Int(25/16) = 1 is the carry and remainder 9 is the unit digit.

Now,

6 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 = 30 + 1 carry = 31

So, Int(31/16) = 1 is the carry and remainder = 15 = F is the digit at 10's place (16's place to be precise)

6 4

3 0

3 6

3 0

3 1

3 2

3 3

3 4

3 5

______________

1 F 9

Ignore carry you will get F9

Now to get complement, subtract each digit by 16-1=15

15 - F = 15 - 15 = 0

15 - 9 = 6

You will get 06

This is 15's complement (255's complement of number)

To get 16's complement, add 1 to it

06 + 01 = 07

This is the result you need