Hello!
To first understand this question, we have to know what a domain is, and what it means to be linear.
The domain is all the possible x values.
Currently, in this graph, it starts at 0, and goes to some point further on.
To be linear means that you have linear growth, which this does not.  This has a bend in it, which is not linear.
So, you must restrict the domain such that you only have a linear section.
If you look at the graph as a whole, it makes a bend. But, if you only look at x value 6 and up, it seems quite straight.
So, this makes sense!
Hope this helps, feel free to ask questions in comments.
-Jeron