Chapter 4.4, Problem 2E

For each of the following subgroups $H$ of the addition groups $Z_{18}$, find the distinct left cosets of $H$ in $Z_{18}$, partition $Z_{18}$ into left cosets of $H$, and state the index $\left[Z_{18}:H\right]$ of $H$ in $Z_{18}$. $H=\langle \left[8\right]\rangle$.