Best Way to Amend Average or Below Average GPA, especially on Mathematics Courses

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Hello everyone,

I'm reaching out to this community for some guidance regarding my application for a Master's in Financial Engineering in the USA. I'm currently in my last semester of undergraduate studies and facing a bit of a dilemma.

My overall GPA is 3.5, which I realize might not be as high as preferred for competitive programs. Moreover, I've had some struggles with calculus and linear algebra courses mostly because they were online, which are crucial for this field. While I've managed to pass these courses, my performance was not as strong as I would have liked.
I'm looking for the best ways to demonstrate my commitment and capability in these areas to the admissions committees. Since I can't change my past grades, I'm considering additional learning options. I'm torn between self-studying through books and online resources or enrolling in formal online courses to show a more structured approach to learning.
I would greatly appreciate any advice from this community:

1. Do you think self-study or formal online courses would be more beneficial in my situation?
2. If you recommend online courses, are there any specific ones that you think would be particularly helpful?
3. How do admissions committees typically view these additional efforts to improve one's knowledge base in specific subject areas?

Any insights, experiences, or recommendations would be immensely helpful. I'm passionate about pursuing a career in financial engineering and want to ensure I'm taking the right steps to achieve this goal.

Thank you all in advance for your time and advice!