hey rajat just have alook at it:
ClO3 ---------> ClO- E= -0.54V **************** 1
ClO- ----------> 1/2 Cl2 E= - 0.45V ***************2
1/2 Cl2 ----------> Cl- E=0.17 V ***************3
ClO3 ------------->Cl- - E=?
eqn 1 + eqn 2 implies
ClO3 ---------> 1/2 Cl2 E= - 0.99V ***************4
1/2 Cl2 ----------> Cl- E=0.17 V ***************3
eqn 4 +eqn 3 implies
ClO3 ---------> Cl- E= -0.82 V
is this your answer ?
if any mistakes then plz let me know