The DOMAIN refers to the input values in any given function, and that is usually represented by the x variable, hence your standard expression
f(x) =
The domain is those values that you input and these determine the other value. In this question the domain includes the values
(30, 1), (30, 2), (42, 2), (55, 3)
You do not have to pick the number 30 twice, hence your answer includes,
[30, 42, 55]