What is the best approach to balance this equation? Why I cannot do it efficiently? What is the best approach to balance this equation?
$$\ce{CO2 + H2O &lt;--&gt; C2H2 + O2}$$

I did it at first this way and I was not successful: 
\begin{align}
\ce{4CO2 + H2O &amp;&lt;--&gt; C2H2 + 3O2}\\
\ce{2CO2 + 6H2O &amp;&lt;--&gt; 6C2H2 + 5O2}\\
\ce{12CO2 + 6H2O &amp;&lt;--&gt; 6C2H2 + 5O2}\\
\end{align}

Should I start from right side or left? They say start from the most complicated molecule? If it is like that, then what is the most complicated one and why?
This may be a bit dumb method, but sometimes I do use it as it is pretty straightforward and doesn't require any guessing. I setup a few equations and try solving over integers.
 $$\ce{aCO2 + bH2O &lt;--&gt; cC2H2 + dO2}$$

Balancing $\ce{C}$ atoms both sides,
$$a = 2c\\$$
Balancing $\ce{H}$ atoms both sides,
$$2b=2c\\$$
Balancing $\ce{O}$ atoms both sides,

$$2a+b=2d$$

Solving gives this. Not hard to see that $a=4$ does the job.