Pay attention on the values of the sources in the circuits to the right: in the upper circuit the voltage source is suppressed, in the lower circuit the current source is suppressed. Superposition principle: a sum of currents at the same components in the circuits to the right is equal to the current in the original circuit to the left where the sources act together (take into account the direction of the currents that are summarized ). Superposition principle holds also for voltages but not for powers.
