Two speakers with opposite phase are positioned 3.5 m apart, both pointing toward a wall 5.0 m

Question:

Two speakers with opposite phase are positioned 3.5 m apart, both pointing toward a wall 5.0 m in front of them (FIGURE 14-47). An observer standing against the wall midway between the speakers hears destructive interference. If the observer hears constructive interference after moving 0.84 m to one side along the wall, what is the frequency of the sound emitted by the speakers?

In figure 14-47