George is at a craft store and sees two types of decorative beads for a project he is working on: blue squares cost about 3.50 per pound, and the red circles cost 1.75 per pound. George wants to buy as many beads as he can with the $21 he has. Write an equation that describes the relationship of the pounds of blue square beads and the pounds of red circle beads that George can buy if he spends all his money. Define each of your variables