Here is the circuit.
What I have to do when there are 2 different sources?
Find all the voltages in the resistors.
Supposing every current going out of the nodes, here are the equations:
-Don't know if the first equation is right, or it has to be: $$-15+(V1-V3)/R1+I1=0$$ -Don't know what am I supposed to do with the \$V1\$ in the NODE3's equation, using -15 instead of \$V1\$ or simply don't put it. (?)
Considering only the NODE2 and NODE3 equations: $$G*V=I$$ I have to find \$G^-1\$: $$G^-1=1/detG*(G^a)^T$$ \$G^a\$ = matrix composed by algebraic complements, \$G^T\$ = transposed.
By doing that I find wrong results.
I know there are many ways to resolve this problem, but I have to practice the node voltage method. Also it can be that I did some algebra or numbers wrong, I'm trying again meanwhile.