for number 4, you just have to add 6 to 5 times more of a number and that equals at least 21. so it would be,
5x + 6 ≥ 21
for number 6 she already has 3 lbs of things in her bag, so you subtact 3 from 18 and get 15 she can carry 15 more pounds, since each book weighs 3 lbs you divide 15 by 3 and get that she can carry 5 more books in her bag.