Isabella buys milk and potatoes at the store.

To find out how much each bag of potatoes costs, let's first define the variables involved in the problem.

Let \( x \) represent the cost of one bag of potatoes.

Isabella buys 3 bags of potatoes, so the total cost for the potatoes would be \( 3x \).

According to the problem, she pays a total of $21.40 for both the milk and the potatoes. She pays $3.73 for the milk.

The equation to represent this situation is:

\[
3x + 3.73 = 21.40
\]

Now, we can solve for \( x \):

1. Subtract the cost of the milk from the total:
\[
3x = 21.40 - 3.73
\]
\[
3x = 17.67
\]

2. Divide both sides by 3 to find the cost of one bag of potatoes:
\[
x = \frac{17.67}{3}
\]
\[
x = 5.89
\]

Thus, each bag of potatoes costs **$5.89**.