Answer:
Ok, we have the function:
y = 2gb*x + 3gb
Where x is the number of streams that you did.
You know that for this function, in a math view, the domain and the range are the set of all real numbers.
(Actually the domain should be integers, but i guess that if you only stream a 0.3534 of a movie and then you quit it, here only consumed 0.3534*2gb, so we can allow x to be a real number)
But in a "real situation", there are some other restrictions:
Restrictions for x:
You can never have a negative value for x (because this has no sense)
So the first restriction is x ≥ 0.
Then, there should be a maximum value of x (you can not stream 4 million things in one month). Or A may be the number of streams that you can watch until you reach the monthly limit of Gbs.
Then the domain is:
0 ≤ x ≤ A.
Now, the minimum of the range is defined by the minimum in the domain:
When x = 0, we have:
y = 2*0 + 3gb
So the minimum value in the range is 3gb.
y ≥ 3gb
And then the maximum value of the range will be when x = A
y = 2Gb*A + 3Gb
3Gb ≤ y ≤ 2Gb*A + 3Gb