Here's why:
* Denotation is the literal, dictionary definition of a word. "Greasy" literally means covered in grease.
* Connotation is the implied meaning or feeling associated with a word. "Greasy" in this context implies danger, slipperiness, and difficulty in maintaining control.
While the racetrack might be literally covered in grease, the sentence suggests something more than just a physical state. It implies a potentially hazardous situation for racers.