What is the domain and range of y=6?

The domain is "what can I make x equal to, and still get a sensible answer. In this case the equation doesnt depend on x at all so the domain is "All real numbers."

The range is "what are all the sensible values the I can get as an answer for y". In this case we are told: "Y always equals six." So the range is just "Y=6".

Some other ways to wirte the same thing:

Domain: {x|x∈R}
Range: {y|y=6}

Domain: (-∞,∞)
Range: {6}