Linear algebra: Everything is happy lad-dee-dah for x and y. The moment we go into matrices I go downhill from there. I saw a very cool youtube on Tesla Dojo and machine learning. I recommend.
Just another example of the huge power in creating accurate digital models of the world.
As an aside, my background is in optimal control/control theory and instrumentation, and operational calculus, state space representations, and matrices were a huge part of the "Jiu-Jitsu" we did. Tinkering with RST control, robust systems, finding optimal systems and sequences (Hamilton-Jacobi-Bellman-Pontryagin) with matrices flying around on our exam papers solving these systems. It was a nice abstraction.
Very, very, powerful tools that command immense respect for Laplace, Lagrange, and people like them who invented things to solve problems we're facing now.