Try this: Add up the given widths of the first 3 bookcases, in cm. This will be a constant. Now write "-2w" after that sum, to represent two more bookcases of unknown widths w. Finally, use the inequality symbol (less than or equal to) and the given wall length to complete this inequality.
Solve it for w, writing your answer as another inequality.
You'd multiply the amount of string for each balloon (12 feet) by the 54 balloons he needs, and you get 648 feet. There are 3 feet in a yard so then you would divide 648 by 3 and get 216 yards
