Lots of ordered pairs lie on that line; in fact, there are an infinite number of ordered pairs that lie on the line. Did they give you a list of particular ordered pairs? But no matter, here's how you can figure it out on your own. Suppose they give the ordered pair (3, 7). That means x=3 and y=7. Plug the x-value into the equation and see if you get y = 7. If you do, then (3,7) lies on the line. If you don't, then (3,7) doesn't lie on the line.
y = 3x
y = 3(3)
y = 9
No, (3,9) lies on the line but not (3,7).
You could also graph the line y=3x then plot the points (ordered pairs) they give you to see if they lie on the line.
