Variable Uncertainty: Solving Equations with Empty Sets

[sillyquark]

Homework Statement

Ok, I don't have a problem with the question, but what I believe to be an empty set variable is used which I have not covered in a course before. My problem is that I do not know the relevance of an empty set in part b) of the equation.

Homework Equations

The equation and original question are attached.

The Attempt at a Solution

 

[sillyquark]

please ignore this silly question, id delete it but I am not sure how