Two passengers are on a moving sidewalk at the airport. passenger a is standing still while passenger b is walking with a speed of 2.0 km/h. the sidewalk is moving west at a speed of 1.5km/h. what is the speed of passenger b relative to an observer standing nearby.

A thing the question doesn't tell us: What direction is Mr. B walking ?

-- If Passenger B is walking West, in the same direction as the moving walkway, then the stationary observer sees him passing by toward the West at 3.5 km/hr.

-- If Passenger B, for some strange reason, is walking BACKWARDS, heading east on the moving walkway, then the stationary observer sees him passing by toward the EAST, at 0.5 Km/hr.

-- We're wondering why the question ever introduced us to Passenger-A.  He never figures into the question in any way.  He has no effect on Passenger-B, and we never hear from him again.