You didnt include any equations, but this information might be what you are looking for.
When asked to used a value for the domain, this refers to the x value. An easy way to remember this is that just as x comes before y alphabetically, domain comes before range also. (x, y) would be first the domain and then the range.
In order for you to have a function, you may have no repeated values in the domain. By looking at the graph of a potential function, you may use "the vertical line test" to determine whether or not it is a function. If a vertical, or up and down, line touches the graph in more than one place then your graph is not a function.