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If you look at each carbon on each side of that double bond, there is a different amount of carbons connected to each one (left of double bond is connected to three carbons, while right of double bond is connected to two carbons). We know that that double bond will act as a nucleophile to attack some kind of electrophile, which is the partially positive H in HBr. Thus, we know that one of these carbons will turn into a carbocation with a positive charge. We always draw the most stable molecule, so, to determine that, we have to look at how many carbons are around each of those two carbons. Ideally, we want to put the positive charge on a carbon that has more carbons connected to it because that way those carbon's attached to the central carbon can donate electrons through the bonds toward that positive central carbon to yield a more neutral charge (inductive effect). Thus, we place the positive charge on the left carbon, since it has three carbons connected to it vs. two. NOW, if we place the positive charge on the left carbon, that must mean we place the hydrogen from the HBr on the right carbon, as that's what this reaction entails (H+ must attach to one of the carbons in this electrophilic addition reaction).
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