Since Vicki wants to have 2 pieces of fruit for every day of the week, she needs 14 pieces. There are 7 days in a week and 2 apples per day, so multiply 7 * 2 to get 14.
Since she only has 6 apples, and she needs 14, subtract 14 - 6 to figure out how much more fruit she needs:
14 - 6 = 8
Vicki needs to buy 8 more apples to have 2 apples for every day of a week.