Help with deciding amplifying factor, small circuit
You can't use the current divider (eqn 3), because \$\small h_{11}\$ is not in series with \$\small R_E\$.

The third independent equation must include \$\small R_2\$ as it hasn't appeared in (1) and (2), hence: \$\small U_{in}=U_{GS1}+(g_mU_{GS1}-I_B)R_2\$ will do the job.