b.  Ta có : " Tổng số đo hai góc kề bù bằng $180^{\circ}$."

=> $\widehat{ABC}+\widehat{ABC'}=180^{\circ}$

<=> $56^{\circ}+\widehat{ABC'}=180^{\circ}$

<=> $\widehat{ABC'}=180^{\circ}-56^{\circ}$

<=> $\widehat{ABC'}=124^{\circ}$

Vậy $\widehat{ABC'}=124^{\circ}$.

c.  Tương tự câu b, ta có :

$\widehat{ABC'}+\widehat{A'BC'}=180^{\circ}$

<=> $124^{\circ}+\widehat{A'BC'}=180^{\circ}$

<=> $\widehat{A'BC'}=180^{\circ}-124^{\circ}$

<=> $\widehat{A'BC'}=56^{\circ}$

Vậy $\widehat{A'BC'}=124^{\circ}$.

Mặt khác : Vì $\widehat{A'BC'}$  và $\widehat{ABC}$ là hai góc đối đỉnh.

=> $\widehat{A'BC'}=\widehat{ABC}=56^{\circ}$.