a) The supply doesn't change because the facility doesn't change, doesn't ramp up or scale back production at all, it just has a new owner. The capital itself is just doing what it was doing before, just under new management. If the new owner expanded and build a new factory, or implemented policy changes that boosted production, THAT would be an increase in supply. Think of it this way, the new supplier enters the market, but the previous one leaves, so there's no net change.
b) An economies of scale is a way of saying that a bigger firm is more efficient at doing it's thing. Wal-Mart is has greater economy of scale than a local store because it's size allows it to do more things at a time in a more efficient way. This contributes to monopoly because it tends to promote big firms, and a monopoly is pretty much the ultimate 'big firm'. There can also be diseconomies of scale, where a bigger firm does some things less efficiently than a smaller firm might.
