derive zeroth lawstatement two systems in thermal

Question

Derive Zero'th Law

Statement: "two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other". 

 Explanation :  If A and B are two systems are equilibrium with system C then T_A = T_C and T_B = T_C.

This implies that T_A = T_B i.e. the system A and B are also in thermal equilibrium.

Therefore we arrived the method of temperature formally through the Zero'th Law.