Answer
$m(x)=3x$
Work Step by Step
The total amount of milk $m(x)$ delivered after $x$ weeks is the product between $x$ and the amount of milk delivered each week:
$$m(x)=3x$$
Since the number of weeks is only a whole number $0,1,2,3\dots$ the domain is the set of whole numbers.
$$\text{Domain}=\{0,1,2,3,\dots\}$$
The range consists of multiples of whole numbers:
$$\text{Range}=\{0,3,6,9,\dots\}$$
As the graph consists of separate points, the function is discrete.
