Electrical Engineering: Principles & Applications (6th Edition)

a. None in series. b. Rx and 2 ohm resistor are in parallel, and 4 ohm and 8 ohm in parallel. c. $R_x=4\Omega$
Use diagram. $v_2=2\times2=4V, v_1=12-v_2=8V$ $i_s=\frac{v_1}4+\frac{v_1}8=3A, i_x=i_s-1A=2A$ $R_x=v_2\div i_x=4\Omega$