Building an incomplete redox half reaction/reduction
It helps to use "fully compressed" formulae so you can easily count atoms and figure out how to balance carbons, oxygens, hydrogens, and charges.  That is, instead of


$\ce{CH3COO- +CO2 -&gt; CH3CH2COO-}$


write it this way:


$\ce{C2H3O2- +CO2 -&gt; C3H5O2-}$


Then it's easy to see that this is not a balanced equation.  Carbon atoms are the same on each side, but not oxygen or hydrogen atoms.  You need two more $\ce{O}$ atoms on the products side.  Those have to come from water since that is the only oxygen-containing species you are allowed to use.


$\ce{C2H3O2- +CO2 -&gt; C3H5O2- + 2H2O}$


Now oxygen is balanced, but hydrogen is still imbalanced.  That can be rectified by adding $\ce{H+}$ into the equation as appropriate.


$\ce{C2H3O2- +CO2 +6H+ -&gt; C3H5O2- + 2H2O}$


Now only charge is imbalanced, which can be fixed by adding electrons to complete the balanced half-reaction:


$\ce{C2H3O2- +CO2 +6H+ +6e- -&gt; C3H5O2- + 2H2O}$