Question

What is the domain of the function y=3 in x graphed below?

Answer 1

That's a natural logarithmic function:
y = Ln(x)
e^y = x

The domain is what values x can be. As you can see from the graph, x > 0

Answer 2

What is the domain of the function y=3 in x graphed below?

Domain is x>0, ln (0) is undefined and there is no negative ln.