Question 141027
I do not have your book, but if you have found the domain and the range, then you can tell if the expression is a function by using the 'vertical line rule'. On a plot of the graph, if any vetical line intersects the graph in two places, that graph is not a function.

In words, if any one value in the domain maps to more than one value in the range, it is not a function.

Basically, a function uniquely maps a single value if X to a single value of Y. You can map many Xs to the same Y, but no single X can be mapped to two different Ys. 

I hope that helps