This looks like a mistake (that's a very suspicious looking 45 degrees). Recall that the expression for the positions of intensity maxima says:
![](https://latex.codecogs.com/png.latex?\bg_white n*lambda=d* sin(theta))
. Look closely at the diagram and you will notice that the paths converge, not at a maximum, but at a minimum. Therefore n = 1.5 (approximately) in this question, and theta = 45 degrees, hence
![](https://latex.codecogs.com/png.latex?\bg_white d=2*lambda)
(approximately). On the other hand, if the angle was 30 degrees, then the answer would come out as exactly
![](https://latex.codecogs.com/png.latex?\bg_white 3*lambda)
. Dodgy question, imho.