We can use these two DNA segments to roughly estimate how many years ago the species shared a common ancestor. First, we can count the number of differences separating segment a from segment b. Because Species B has 5 differences when compared to Species A, we see that as 5 different mutations that happened, or errors when copying the DNA. If there was one mutations per 15 million years, this means these two species shared a common ancestor around 75 million years ago. Hope I helped!