In college, I took Linear Algebra with Differential Equations, and did not do that well in it. As an adult, I realized that I got confused between the abstract part and the applied part and things just seemed very disconnected to me.
Two things in particular confused me. One was that - as a physics major - I was expecting to find solutions that were much more exact, so the problem was “solved” before I thought a solution had been specified, so I was confused. I realize later that the issue of solving for the boundary conditions was usually left out of the problem, and when it wasn’t, that was almost as much work (or more) as the linear algebra part. So I could do the steps, but I didn’t seem to know when I had finished, and just asked myself “wait, what did I forget to do?” Which left me thinking I didn’t understand any of it (which arguably was the case).
The other thing that bothered me was that I thought I was going to learn to solve more types of differential equations, and not just the equations in the space of polynomials, which all seemed to have solutions of the type e^x. I suspect the insight that polynomials are a vector space just wasn’t illustrated well enough. I was thinking in terms of geometry, so the mapping of the geometry to the space of polynomials somehow got lost. But also, I thought that the types of equations I was likely to encounter in physics might not be specifiable as simple polynomials (ironically, one does find more of these in economics than in physics). I didn’t realize at the time that physicists get around some of these by specifying things in terms of e^ix and solving with that solution.
I had to revisit linear algebra later in life when I was looking at statistics for data analysis, and it started to make more sense then. Plus someone gave me a better explanation of how polynomials and vector spaces relate and it started to fall better into place.
My linear algebra and differential equations class was at 8 AM, 4 days a week, as far away from my apartment as you could get on campus. Long story short, I never got to find out if I understood any of it because I never went. That was $1500 well spent.
And who throws away perfectly good Scotch?! 
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