2.35c+5.50d=89.5
Since the cats cost $2.35, substitute c for cats and multiply by 2.35 to find the amount spent for a given amount of cats. Dogs cost $5.50, which is where 5.50 d c0mes from. Since you're looking for a total, you would add the two variables. Then, since you're given a total of 89.50, you set the equation equal to that total.