In basic medium balance the half-reactions following below:
`Cl_(2)O_(7)toClO_(2)^(-2)`

Explanation

In order to balance the number of O-atoms on both sides ,3 molecules of `H_(2)O(l)`are added to the left side (because there is excess O-atoms on this side) and for these three `H_(2)O(l)` molecules ,six `OH^(-)(aq)` ions are added to the right side .Then we get -
`CI_(2)O_(7)+3H_(2)Oto2CIO_(2)^(-)+6OH^(-)`
For balamcimng the charge ,8 electrons are added to the left side .Hence ,the balanced equation is. `CI_(2)O_(7)+3H_(2)O+8e to2CIO_(2)^(-)+6OH^(-)`