Answer
$\frac{d}{\lambda} = 7.5$
Work Step by Step
We can assume that the vertical line which bisects the two sources is the y axis. At all points on the y axis, the path length difference is zero, so the phase difference is zero. At all points on this vertical line, both above the x axis and below the x axis, there will be a maximum. Note that there is no minimum on the y axis.
Since there are a total of 30 minima (points of zero intensity), by symmetry, there must be 7 minima in each quadrant.
These points have a path difference of the form: $(m+\frac{1}{2})~\lambda,$ where $m = 0,1,2,3,4,5,6$
Then the points on the x axis to the left and to the right of the sources must have a path length difference of $7.5~\lambda$
Therefore, the distance $d$ between the two sources must be $~~d = 7.5~\lambda$
Then:
$\frac{d}{\lambda} = 7.5$
