#3 represents a linear function; it's obvious that the graph is a straight line.
#4 represents a linear relationship for the same reason. However, this is NOT a function, because if we choose x=2, there are an infinite number of y values.
Keep in mind that "only one y value for any given x value" is a requirement when you are selecting "functions."
Please try doing one of the other problems on this worksheet yourself. Then I'd be glad to give you feedback on your work.