and my solution

calculations for A,B,C (1.1)

\(-I_1 +I_3 + I_5 = 0\)

\(-I_3 + I_4 - I_6 = 0\)

\(I_2 -I_4 -I_6 = 0\)

calculations for I, II, III (2.1)

\(-U_1 -U_3 +0 = 0\)

\(-U_2 -U_4 -0 = 0\)

\(U_3 + U_4 + 25 = 0\)

because \(U_6=0, E_5=25V\)

i use resistors \(R_1,R_2,R_3,R_4\) in this formula \(U_i=R_iI_i\)

and i have got (2.2)

\(-2I_1 -8I_3 = 0\)

\(-1I_2 -9I_4 = 0\)

\(8I_3 + 9I_4 + 25 = 0\)

i try to calculate from 1.1 and 1.2 this current \(I_6\) (i thinks this is what i need to find), but i cannot obtain the result. by the way calculations are too complicated for an exercise like this

greetings!