3-Phenylpropene on reaction with $\text{HBr}$ gives (as a major product):

According to Markovnikov's rule, the negative part of an unsymmetrical reagent (like ${\text{Br}}^{-}$ from $\text{HBr}$) adds to the carbon atom of the double bond that is more substituted.For 3-phenylpropene (${\text{C}}_6{\text{H}}_5{\text{CH}}_2\text{CH}={\text{CH}}_2$):The addition of $\text{HBr}$ follows the rule, but stability of the intermediate carbocation is key. The major product is 1-bromo-1-phenylpropane (${\text{C}}_6{\text{H}}_5{\text{CH(Br)CH}}_2{\text{CH}}_3$) because the intermediate benzylic carbocation is highly resonance-stabilized.