How did today's lab question relate to double slits?

I think what you should have seen in your lab is a double slit with a finite width. If you have two finite-width slits, you'll have a narrow fringe pattern due to interference and an ``envelope'' due to the diffraction of the finite width slits. You can use the minima of the envelope to estimate the width of the slits with $\sin\theta_{\rm dark} = m \lambda/a$.