Find the value of U by the means of superposition
With \$E\$ shorted out, you have \$R\$ in parallel with \$2 R\$. The \$J\$ current will divide between these. Select only the portion that goes through \$R_0\$ here and write it down.

Now return \$E\$ and remove (open) \$J\$. Now you have \$E\$ in a series loop containing two \$R\$ resistors and one \$R_0\$ resistor. Again, compute the current through \$R_0\$ here and write it down.

Now sum the two values. What do you get?



P.S. It may help a little that I also suggest you look at the direction of the current arrow for \$R_0\$. This may indicate a sign reversal to be thinking about.