That's a great question ashley4chencho and thanks for posting.
Is this a subscription directly from Google Play Music? If so, can you give this link a shot and see if you can cancel it? Let me know. Thank you.
If it's a subscription through Google Play Store.
You should have a tab saying subscriptions.
Open Google Play
Tap the Action menu (upper left - looks like 3 lines)
Tap my Apps
There you should see 3 tabs and one is subscription.
From there you can cancel your subscriptions.
If it's YouTube Red or Google Play Music subs you can do it from their apps.