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