Two's complement 8-bit subtractor? Please help me! I've been asked to make a circuit using only Logic Gates to implement an 8-bit 2's complement subtractor. 

I was thinking of somehow modifying an 8-bit adder, but I figured it isn't 2's complement so it won't be of much use to me.

Any help is greatly appreciated, even if it's a pointer to the right approach to take in this.
It may seem at first as though computing the two's complement of b would cost you a whole separate incrementer, but you can easily merge it into the adder that you need anyway: just set the carry-in.

Alternatively (for example if you are somehow forced to use an adder without carry-in, or if it just works out better in combination with other logic) you may used that a - b = ~(~a + b), which obviously only ever needs inverters and a normal adder. This does put inverters before and after though, whereas to calculate the two's complement of b you'd only need inverters before the adder (and a carry-in but that's almost free).