When professor Gilbert Strang of MIT throws a few matrices up on his chalkboard (https://agentanakinai.wordpress.com/2019/04/01/linear-algebra/), I pause the video and I try to work out the solution myself. I then use this Linear Algebra Calculator app to check my result, and potentially rework my calculations, before resuming the video.

As you can see, I did a very simple calculation here. My point is that this app is very quick and easy to use. If you still have any difficulty, professor Strang goes on to explain everything, of course. But, the more of his lectures I watch, the easier these calculations become. This just helps to prove, obviously, how good a professor he is (https://agentanakinai.wordpress.com/2019/04/04/gilbert-strang/).