 So if you asked Alice where both of these events happened she'd say well I was just standing along the origin the whole time. So delta xa the difference in position between these two events is zero because they both happened at the same place in Alice's coordinates. So what about in Bob's coordinates? Bob measured some time delta tb between these two events and in that time Alice was traveling at a speed v. So the total spacing between these two events in Bob's frame is v times delta tb. Lastly how do delta ta and delta tb relate? So either delta ta equals gamma delta tb or delta tb equals gamma delta ta. If you remember back to when we derived the time dilation formula we did it by having Bob on a platform watching Alice on a moving train holding a light clock. From Bob's point of view the ticks of the light clock happened in different locations whereas from Alice's point of view the ticks happened in the same location and it was under this situation that we derived delta tb equals gamma delta ta. Here the situation is similar. Both events happen in the same place in Alice's frame but in different places in Bob's frame and it's under this situation that we have delta tb equals gamma delta ta. So we now have a few different relations that we can stick into a Lorentz transforms. So if you substitute these in and play around you should be able to solve for d and f.