See attached file.
At junction B, by KCL (Kirchhoff's current law):
sum(Ii) = 0 => I1+I2 - I3 = 0
=> I3 = I1 + I2 .... (1)
For closed loop ABEFA, by KVL (Kirchhoff's voltage law):
sum(Ii*Ri) = sum(Ei) (take clockwise as +ve direction):
I3*R4 + I1*R1 = E1 ..........(2)
For closed mesh or ...
The current is found using Kirchhoff's current law. The flows through each of the four resistors in a figure are given.