In part a an AC generator labeled E1 is connected to a 50 ohm resistor. That's it. In part b there is an AC generator on the left which is labeled E1. The top of this generator is connected to a 50 ohm resistor. The far end of this resistor is connected to the top of another AC generator. The wave inside the circle which represents the generator indicates that it has the opposite phase as the one on the left. This generator is labeled E2 = minus 50 times E1. The bottom ends of the two generators are connected together.