Give a balanced equation for the reaction of C3H6 with bromine and state one observation that can be made during the reaction?

The balanced equation is C₃H₆ + Br₂ → C₃H₆Br₂.

You do not state any special conditions, so I assume that this is a thermal rather than a photochemical reaction.

You know that this is not an alkane, because its formula is not C₃H₈ (##C_nH_(2n+2)##).

It is missing two H atoms, so it must contain either a ring (cyclopropane) or a double bond (propene).

Cycloalkanes do not react readily with bromine, so the compound must be propene.

The reaction is the addition of bromine to a C=C double bond.

(using structural formulas) or

CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br (condensed structural formulas) or

C₃H₆ + Br₂ → C₃H₆Br₂ (molecular formulas).

You will see the reddish brown colour of bromine disappear as rapidly as you add it to the reaction mixture.