Ewww... yeah that's a tricky one.
So seldomly am I concerned about calculating a CPA that I sorta didn't build any shortcut methods for auto-plotting a DRM line given two ship locations from a DRT plot.
Usually the DRM is observed based on M1 and M2 and you just connect the line and the CPA forms a right triangle to the DRM line. In this case they're just telling us we're gonna be coming by your area and we only know the M1 position. We have to determine the DRM line. I don't have any handy tools for doing that automagically... at least not yet.
Here's one way to plot the DRM so you can determine the CPA...
Shoot, I wouldn't of figured out that one without reading their steps and looking at their example plot on page 11-19. So don't feel too bad...