Sum of the oxidation numbers of carbon in acetaldehyde is     

The formula of acetaldehyde is ${\mathrm{CH}}_3\mathrm{CHO}$

The oxidation states of hydrogen and oxygen are $+1$ and $-2$.

The oxidation state of carbon attached to $3$ hydrogen’s is $-3$ 

The oxidation state of carbon attached to $1$ hydrogen and $1$ oxygen is $+1$

So the sum of oxidation state of carbon is $-3+1= -2$