I3=I2 + I1
I2=I3 - I1=1A - 2A= - 1A
-E3+E2=-I3(R3 + r3) - I2(R2+r2)
-E1+E2=I1(R1+r1) - I2(R2+r2)
E2= - I3(R3+r3) - I2(R2+r2) + E3= -1(6+1)-(-1)(4+1)+20= -7+5+20= -2+20=18 (V)
-E1=I1(R1+r1) - I2(R2+r2)-E2 | x(-1)
E1= -I1(R1+r1) +I2(R2+r2)+E2= -2(2+1) -1(4+1)+18= -6-5+18= -11+18=7(V)