$${\text{CH}}_3{\text{CH}}_2{\text{COO}}^{-}{\text{Na}}^{+}+\text{NaOH}\stackrel{\text{Heat},?}{\to }{\text{CH}}_3{\text{CH}}_3+{\text{Na}}_2{\text{CO}}_3$$
Identify the missing reagent:

This is the Soda lime decarboxylation reaction. Sodium salts of carboxylic acids are heated with soda lime (a mixture of $\text{NaOH}$ and $\text{CaO}$ in $3:1$ ratio) to form an alkane with one less Carbon atom than the parent acid. $\text{CaO}$ keeps $\text{NaOH}$ dry and porous.