Coherent monochromatic light of wavelength \(\lambda\) is incident on two parallel slits, each of width \(a\), separated by center-to-center distance \(d\). The resulting intensity pattern on a screen far from the slits is shown in the graph of relative intensity \(I/I_0\) as a function of position \(x\). Based on the graph, what is the ratio \(d/a\) of the slit separation to the individual slit width?