since the smallest possible prime number is 2 it would make since if each side is 2 which makes the smallest perimeter possible. i really hope this helped :)
She bought 42 more cans of cat food becuase 6 packages with 9 cans is 6 times 9 and that is 54 -42 to equal 12 because 3 times 4 which is how much dog food she buys