[wail_poet]

steps
====
when K is open R total = V/I = 6 ohms
R1+R2+R3 = 6 ================>1

when K is on A so there is a branch in R2 and R2
R tot = V/I = 6/1.2 = 5 ohms
R1 + 0.5R2 + R3 = 5 =============>2
اطرح المعادلتين
0.5 R2 = 1 so R2 = 2 ohms