simply amazing explanations. Thank you so much.

Just a side note: @ 34:30: The KCL is: ((Va - Vb) / R1) + 3Vx = Vb / R3 A KCL must always have the unit of current [A]. Therefore the number 3 must have the unit [A/V] here.

For the last exercise, before using supernodes, only have 3 equations for 4 unknowns, but why don't use Va+Vx=Vb now? So we get 4 equations for 4 unknowns, don't need supernodes anymore. Is that make sense?

great

The last exercise could be simplified to one source and one resistor, given that R3 and R4 are in series, and then they are in parallel to the R2, and this is also in series to R1... Therefore, with one resistor and one voltage source Ix could be simply calculated. Or do I miss something?