Answer:
The plan is:
A fixed cost of $50 per month.
An extra of $15 per GB used over the limit of 2B.
We can write this as a linear equation:
C(x) = $15*x + $50
C(x) is the monthly cost, and x is the number of GB that she used over the limit.
Then, if she used 3GB over the limit, we should replace x by 3.
C(3) = $15*3 + $50 = $45 + $50 = $95.
And the answer to the second question was already found, if she used xx GB in the month, then we have:
C(xx) = $15*xx + $50
(we replaced the x in the general equation by xx)