In this question, you have to determine the resultant wave - that is, the result of the interference of the two individual waves. To do so, you will have to determine the displacement of the medium at each of nine locations. Once you determine the displacement, drag the grey dot (above the diagram) to the location represented by that displacement. Once done successfully, the dot will snap into place. Here's how to determine the displacements:
When two waves travel along the same medium, each wave exerts its own individual effect upon the medium. That is to say that each waves either pulls upward ("positive" direction) or pulls downward ("negative" direction) upon the medium by a given amount. But the medium can have only one response. It can't show both waves; it must show the combined effect of both waves. And this combined effect is easily determined by simply adding the two individual effects. This is known as the Principle of Superposition.
To determine the effect at each location, simply add the displacement of both waves at that location. That is, measure (or count) upward (or downward) to the red wave to determine the displacement of the medium that would result from the red wave acting alone. Repeat this measurement (or count) for the blue wave. Add the two numbers together and the result is the combined displacement at that location for the two waves acting together. Note that one or both of the numbers that you are adding together could be negative (for downward).