Question

design combinational circuit whose input is a 4 bit number and output is the 2's complement of the input number

Answer 1

What is your problem with that ? I can help you if you want

Answer 2

This figure shown to you how design 2's complement in some way and you can figure out another way if you understand basic concept of logic design

Any help i will be ready

Answer 3

let a3 a2 a1 a0 => input
t3 t2 t1 t0 => output in 2's complement form

so
t0 = a0

t1 = t0 xor a2

t2 = (t0 or t1) xor a2

t 3 = (t0 or t1 or t2) xor a3

Construct the circuit equivalent of the logic expr above..... u will get ur ans