Well, velocity (rate) equals distance / time. (One can remember this easily by noting that speed is measured in "miles per hour" or "km per hour" if unlike me you live in a rational country.) So 14 = Janet's distance / 7.5, or Janet's distance = 14*7.5.
Now, whoever wrote the textbook could use a refresher course on communication skills, because the problem is posed fairly awkwardly. But it seems that one should just subtract 12 from Janet's distance to find Jack's distance. Who cares how fast he rode? I guess it is a red herring.