Given info:

Time(s). (ClO) mol/L
0.0. 0.012
4.0. 0.008
8.0. 0.005
12.0. 0.0035
16.0. 0.0025
20.0. 0.002
24.0. 0.0018
28.0. 0.0017

Calculate the average rate of the overall reaction

Wouldn't that be (0.012-0.0017)/28 = ?