3 - Methylhex-2-ene on reaction with HBr in presence of peroxide...

The hydrogen bromide addition in peroxides follows Anti Markovnikov rule. Anti Markovnikov  addition reaction is found to follow a free radical mechanism. The peroxide compound involved helps in the generation of free radicals. The Anti-Markovnikov rule defines regiochemistry in which the substituent is attached to a less substituted carbon instead of the more substituted carbon.

The reaction of $3 -$Methylhex-2-ene  with $HBr$ in presence of peroxide, takes place as follows,

$2$ chiral carbons, here, so, total number of stereoisomers will be ${2}^{n} = {2}^{2} = 4$