Here's why and how to balance it:

Unbalanced:

* Carbon: There are 3 carbon atoms on the left (C3H6) and only 1 on the right (CO2).

* Hydrogen: There are 6 hydrogen atoms on the left (C3H6) and only 2 on the right (H2O).

* Oxygen: The number of oxygen atoms is not equal on both sides.

Balanced Equation:

To balance the equation, you need to adjust the coefficients in front of each molecule:

2 C3H6 + 9 O2 → 6 CO2 + 6 H2O

Explanation:

* Carbon: Now there are 6 carbon atoms on both sides.

* Hydrogen: Now there are 12 hydrogen atoms on both sides.

* Oxygen: Now there are 18 oxygen atoms on both sides.

Key Point: Balancing chemical equations ensures that the law of conservation of mass is upheld, meaning the same number of atoms of each element is present on both sides of the reaction.