Network Analysis problem
Because the networks are linear you can apply the superposition theorem. Let each network have an output x to an input of 1V. The output to an arbitrary input is then:
$$V_o = V_i x$$
Applying the superposition theorem for the two sources:
$$V = ( V_1 + V_2 ) x$$
Taking into account that there are no internal sources in the networks.
For the input of 1V and 3V one gets
$$4x = 6 \rightarrow x = 1.5$$
You can then calculate the voltages.