Create an Unlock Key using a Web Page
When you find that GitHub repository, don't just git clone and submit. Copy the code into a Jupyter Notebook. Change the initial conditions. Plot the result. If you can break the code and fix it again, you have mastered numerical methods.
Searching for a direct answer key might help you get a certificate, but it won't help you in a technical interview or on the job. Engineering firms look for people who understand a specific method was chosen. If you are stuck on a specific problem: numerical methods for engineers coursera answers