# What alkene should be used to synthesize 3-bromohexane? How can I explain the carbocation process?

You can start with 3-hexene. Since the molecule is symmetrical through its double bond, it doesn't matter whether you add $B r$ to carbon 3 or 4. You will get the same thing. Neither carbon (3 or 4) has more protons than the other.
From here, all you need to know is that since there is only one $- B r$ on the product, it must be a hydrobromination of the appropriate alkene.
${H}_{3} C C {H}_{2} \left(H C = C H\right) C {H}_{2} C {H}_{3} + H B r \to {H}_{3} C C {H}_{2} \left(H C B r - C {H}_{2}\right) C {H}_{2} C {H}_{3}$
The carbocation intermediate forms after the nucleophilic alkene donates its $\pi$ electrons to grab the proton from $H B r$ (the electrophile), and then $B {r}^{-}$ acts as a nucleophile and donates electrons to attach onto the carbocationic center.