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 (
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.