Kirchihoff's Law is very fundamental law in the circuit analysis.
Kirchihoff's Law has two laws: at the junction, and in the closed circuit.
1.KCL (current law)
At the any junctions,
∑I=0
2.KVL (voltage law)
In the closed circuit,
∑V=0
Example:
How much current does flow through R4?
V1 and V2 are DC Voltage source.
I'll tell you the solution tomorrow.