How do you write a balanced equation for the complete combustion of #C_3H_6O#?
Add coefficients in front of each species so there is the same number of atoms of each element on both sides of the equation. Never change subscripts.
Complete combustion of an organic substance requires oxygen and produces carbon dioxide and water.
Notice that there are 3 carbon atoms on both sides, 6 hydrogen atoms on both sides, and 9 oxygen atoms on both sides.
Also notice that no chemical formula was changed (no change in subscripts.