y = 0 when sin = 0
i.e., when 2.7p - 6p x = 0
x = 2.7/6 = 0.450 m

Of course y is also 0 at every wavelength multiple (1/3 m see part b) on either side of this point
i.e., 0.450 - 0.333 = 0.117 m
0.450 - 2(0.333) = -0.216 m

Is y increasing or decreasing as x increases by D x at these points?

answer