R1 = R2 = R3 = 2 Ω
R4 = R5 = R6 = R7 = 4 Ω
R12=R1+R2
R12=2+2=4 Ω
R123=(R12*R3)/(R12+R3)
R123=(4*2)/(4+2)
R123=(4/3) Ω
R1234=R123+R4
R1234=(4/3)+4
R1234=(16/3) Ω
R567=R5+R6+R7
R567=4+4+4=12 Ω
Rz=(R567*R1234)/(R567+R1234)
Rz=(12*(16/3))/(12+16/3)
Rz = (48/13) ≈ 3,7 Ω