balance CH3-CH=CH-CH3 + Br2------?

Answer 1

This is a specific type of electrophilic addition reaction called halogenation. You are adding the Br2 across the double bond through anti-addition. The mechanism of the reaction is as follows. The carbon double bond will attack one of the bromines and form a cyclic bromonium intermediate. The other bromine will either attack the 2 or 3 carbon and break one of the bonds of the cyclic bromonium intermediate. The final product is 2,3-dibromobutane.

Answer 2

CH3-CHBr-CHBr-CH3

2,3-Dibromobutane