What is the difference between a relation and a function?
A function y = f(x) is a relation of x and y such that for each value y value there is a unique x-value. A relation can be such that for a particular value of x there is more than one value of y that satisfies the relation. This is not the case.
For the function y = f(x), there is only one value of y given by every value that x takes.