1 subscription = $21
28 subscriptions = x
You know 1 subscription is $21
You want to find out how much 28 subscriptions are
Multiply 28 (subscriptions) by 21 ($ per subscription)
28 x 21 = 588
28 subscriptions cost $588
If you would like the formula to find out how many subscriptions cost hat amount of money,
21x = y
x is how many subscriptions you want, while y is the total cost of the money