Answer
$2C_3H_6(g)+9O_2(g)\rightarrow 6CO_2(g)+6H_2O(g)$
This is combustion.
Work Step by Step
$C_3H_6(g)+O_2(g)\rightarrow CO_2(g)+H_2O(g)$
Is a combustion reaction because a hydrocarbon and oxygen react to form carbon dioxide and water.
Add a coefficient of 3 to $CO_2$ to balance $C$.
Add a coefficient of 3 to $H_2O$ to balance $H$.
$C_3H_6(g)+O_2(g)\rightarrow 3CO_2(g)+3H_2O(g)$
The number of $O$ atoms is odd on the right and even on the left. Multiply all coefficients by 2 to eliminate this situation.
$2C_3H_6(g)+2O_2(g)\rightarrow 6CO_2(g)+6H_2O(g)$
Increase the coefficient of $O_2$ from 2 to 9 to balance $O$.
$2C_3H_6(g)+9O_2(g)\rightarrow 6CO_2(g)+6H_2O(g)$
