A question about superposition and zeroing condition
You have a voltage source E and a current source I. We will calculate the effect of individual sources on Ux. Ux is the voltage drop across the resistor when a current is passed through it, from left to right. Let that current be Ix. 

The figure shows the circuit after removing the current source. 



From figure, $$Ux_1 = E\times \frac{R}{R+R} = E/2 \tag1$$

Now, removing the voltage source,



$$Ix = -I\times \frac{R}{R+R} = -I/2$$
$$Ux_2 = IxR = -IR/2\tag2$$

Now \$Ux = Ux_1 + Ux_2 = \dfrac{E-IR}{2}\$

Now Ux can be made zero if E = IR. This is your zeroing condition.