Answer
4 V.
Work Step by Step
For two or more devices connected in series, the supply voltage (here, it is 6 V) equals the sum of the voltage drops across the devices. The voltage drop (i.e., voltage difference) across the first lamp is 2 V, so the voltage drop across the second is 4 V.
This is Rule 4 for series circuits, on page 443. Series circuits are discussed on pages 442-443.
