What relation is valid for the voltage U6 in the adjacent circuit?
Just go in a circle (arbitrary direction) and add all voltages together. If the arrow is pointing in the other direction, subtract, For example

\$ U_6 + U_5 + U_4 - U_3 + U_2 - U_1 = 0\$

It doesnt really matter which direction you go, or which arrow direction you chose to count positive, as long as you do it consistently.

When you solve this for \$U_6\$ you get the correct answer (which is 2).