It's quite easy really it may look confusing, but all you have to do is look at the number line it says a to b to c well that means that you have already traveled 9km so then you travel to b again so count the little lines: we already have 9km so now count, 10 11 12 13km, then go back and add 4 again since you have to go back to c: 14 15 16 17km, and there's your answer, hope this wasn't confusing for you I tried the best I could!
