Carmela currently has 14 boxes of 315 cards. 14 multiplied by 315 will give you a result of 4410.
Her estimate, 3000, is off by 1410.
She buys six boxes that hold 500 cards each. 500 multiplied by 6 will give you 3000 card spaces.
Therefore, she will not have enough room. If her estimate, 3000, was correct, she would then have enough room. However, she has 4410 cards, so she will have 1410 cards without cases.