If you subscribed to Grokker through the Google Play Store your subscription is managed by Google. Therefore, you must cancel your membership through Google. You can do this on your mobile device with the following instructions:
1. Open the Google Play Store app.
2. Click on the button in the upper right corner to access your google account.
3. Select "Payments & subscriptions".
4. Select "Subscriptions".
5. Find Grokker in the list of your subscriptions, and you'll be able to unsubscribe from here.