While walking along the shore at your beachfront home, you notice that there are two narrow gaps in the breakwater, the wall that protects the shore from the waves. These gaps are 9.0 m apart and the breakwater is 12.0 m from the shore and parallel to it. You go to the shore directly opposite the midpoint between these gaps. As you walk along the shore, the first point where no waves reach you is 1.7 m from your starting point. Out beyond the breakwater you observe that there are ten wave crests in 18 s. To me, this seems to be a straightforward application of the formula r2-r1 = (m+1/2)lambda. I found the path difference (r2-r1) by simple geometry (1.19m), then I set m=0 since 1.7m is the first point of destructive interference. The answer I have is lambda = 2.38m, and it is wrong for some reason. I also do not understand the significance of the given frequency, and I have not used it in my calculations. I would love to know what I'm missing here. Thanks!