By definition, $z_{21}=V_2/I_1$ with the output port open, I2=0. The direct 4 Ω path from port 1 to the centre is parallel to the upper 2 Ω plus right 2 Ω path, also 4 Ω.
Input current divides equally between these paths and then all passes through the centre-to-ground 2 Ω resistor. The centre voltage is 2I1. The output is above it by $(I_1/2)(2)=I_1$.
Thus V2=3I1 and **$z_{21}=3\ \Omega$**.
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