Currently, as an assistant buyer she earns $472 each week. With 52 weeks in a year we can do 472*52 to get her current salary of $24544 per year.
Once she is promoted, she will be paid $1,200 2 times a month so that is 24 times a year and 1200*24= $28800 because semimonthly means 2 times per month.
With this promotion she will be earning $4256 more per year than before.