TITLE: Sanity check, is $\{(-9,-3),(2,-1),(7,7),(-1,-1)\}$ a function? 
QUESTION [6 upvotes]: EDIT#2: Yes, I'm crazy! This IS a function. Thanks for beating the correct logic into me everyone!
I'm using a website provided by my algebra textbook that has questions and answers. It has the following question: 
Determine whether the following relation represents a function:
$$\{(-9,-3),(2,-1),(7,7),(-1,-1)\}$$
I answered NO, it is not a function but the website says it is. Am I wrong? If so, what am I missing?
EDIT: I was given the following definition in class: 

Function: A function is a rule which assigns to each X, called the domain, a unique y, called the range. 

My instructor also said that if you plot the points you can tell if it is not a function if it fails the vertical line test. Here is the graph of the above points, and for example it would fail the vertical line test if I drew one on x = 1, right?

Thanks!
Jason

REPLY [2 votes]: One way to precisely define a function is as follows: A function is a collection of ordered pairs, no two of which have the same first term. From this definition, it is immediate that your collection of ordered pairs is a function.