Over all I found the chapter on relations and functions to be quite and easy and understandable unit. The section I found most challenging was being able to identify a function, this s how to do it:

This image represents two diagrams, the diagram on the right represents a function relation and the diagram on the left does not (it is still a relation but no longer considered a function). The way we can tell which is which is by looking at the input, for a relation to be a function each input may only map to one output, note, this isn’t to say that more than one input can be mapped to the same output, but each input must be mapped to only one output (as shown in the diagram on the right). To identify weather or not a relation is a function by looking at a graph you can do something called the “vertical line test”, this is where you draw vertical lines through the graph and see if there are two or more contact points on each line, if no lines go through more than one single point, the relation is a function, if not then it is not a function.