Answer
$C_8H_8(l)+10O_2(g)\rightarrow 8CO_2+4H_2O$
Work Step by Step
Combustion means $O_2$ is the other reactant.
The unbalanced equation is:
$C_8H_8(l)+O_2(g)\rightarrow CO_2+H_2O$
Add a coefficient of 8 to $CO_2$ to balance $C$.
Add a coefficient of 4 to $H_2O$ to balance $H$.
$C_8H_8(l)+O_2(g)\rightarrow 8CO_2+4H_2O$
The right side now has 20 $O$ atoms. Add a coefficient of 10 to $O_2$ to balance.
$C_8H_8(l)+10O_2(g)\rightarrow 8CO_2+4H_2O$
