The 5-pound bag is better value.
The easiest way to compare and find out which is a better buy is to find out how much is being charged for 1-pound of food in each bag.
$12 is divided by 3 beacuse it's a 3-pound bag and we are aiming to find what 1-pound is equal to.
$12 ÷ 3 = $4
So the first bag charges $4 per pound of food.
Then use the same method for the 5-pound bag.
$14.25 ÷ 5 = $2.85
So the second charges $2.85 per pound of food.
So as we can see, the 5-pound bag is clearly better value. For each pound of food you are saving $1.15 in comparison to what you would be spending if you opted for the first bag.
Hope this helps!