It's the old-but-important concept: "Speed" depends on who's measuring it and how THEY're moving. Different observers may very well observe different speeds, and they're all correct. There's no such thing as "the REAL speed".
When you were a little kid, did you ever get on a moving escalator or walkway, and when you got to the middle, you turned around and walked the <em>opposite</em> way, so that somebody watching you from the outside would see you not moving at all ?
Say you're on a school bus that's driving 10 mph along the street pavement, and you get up out of your seat and run forward up the aisle at 10 mph. Somebody outside the bus sees you passing them at 20 mph !
If instead, you run toward the BACK of the bus at 10 mph, somebody outside the bus sees you bobbing up and down but not moving forward or backward at all.
All this problem is saying is: The bus is driving along at 15 m/s. A passenger on the bus puts his little baseball down on the floor on its little feet, and it runs backwards down the aisle, toward the back of the bus, at 15 m/s. Somebody is standing outside looking into the bus as it passes by and the little baseball is scurrying toward the back of the bus. How will HE describe the motion of the baseball ?
Have you got it now ?
"Relative to the Earth" just means how fast an object is passing the stores and telephone poles and people standing still ... things that are attached to the Earth. "Relative to the bus" would mean how fast an object is passing by people sitting on the bus.
