You should use option 2. divide $8.40 by 6.
Six pillows for $8.40 means that $8.40 is the total amount you pay for buying six pillows. however, if you buy only one, you need to divide the price of $8.40 by 6 to get the price of each pillow.
$8.40 / 6 = $1.40 price of each pillow.