The floor of the cage is 15 square feet (3*5=15) and since there is not enough shavings in one beg to cover that area and two is a little bit too much, you still need more than 1 bag. And thus the answer is two bags.
using a second for it, since it's a horizontal line, then say hmmm we'll use the y-intercept, or (0, 1), so the equation of a line that passes through (2,1) and (0,1)