2-Bit Adder / Subtractor

Remember that to subtract 2 binary numbers (in 2s compliment) you just flip all the bits of the second number and add 1. So you should be able to create a circuit that does this.

You will have 4 inputs for the numbers and 1 control bit. When it is 0, you are adding. When it is 1 you are subtracting. You will also still have 2 bits as the output and another bit that is the "overflow"

This might help. We will not be using "multiplexers" but the same outcome can be done with XOR gates.