How to get a refund for Brainly free trial incorrect charge?

If you haven't already, cancel the subscription so that you wont't be charged again when it renews in the future: If you want to cancel a subscription from Apple - Apple Support

Subscriptions aren't usually refundable but you can try the following: Request a refund for apps or content that you bought from Apple - Apple Support

Brainly also has a subscription that you can sign up for a free trial directly on their website. It is common to be directed to websites for subscriptions when they come from links in Social Media posts or website advertisements. Any subscription made on a website directly bypasses Apple's ability to manage the subscription and you will not see it in the Subscription section on your device.

For any subscription that is made with a company directly, Apple has no control over it, so you will have to deal with Brainly to cancel the subscription and also request a refund through them as well. Brainly gives this information to cancel a subscription that was made on their website.

https://faq-us.brainly.com/hc/en-us/articles/13227656874002-Brainly-Web-FREE-TRIAL

If you believe you were charged fraudulently then that would need to be disputed with the Credit Card that was used.