I have no appropriate top-level comment because I cannot remember learning this in diffeq either. For this kind of question, *you should talk to the instructor for the course or another in the department*, someone who is paid to help students, not random people online who are answering questions for fun.
Just from Google searching I did find this https://www.mathsisfun.com/calculus/differential-equations-variation-parameters.html
It sounds like you’re taking the problem of interest, cutting out some of the specifics so that it looks like a system that has a general solution of set of solutions, solving that, and then adding back in the part you took out to get a specific solution.
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