we assume that "one tenth of its length per week" is interpreted as "one tenth of the total length is completed each length", rather than "one tenth of what remains is completed each week"

the kilometers completed each week is 1/10 of the total:

17 / 10 = 1.7

1.7 km per week, after 6 weeks we have completed:

1.7 * 6 = 10.2

17-10.2 = 6.8

6.8 km remain but we want to know how many meters remain, so:

6.8 km * 1000m/km = 6800 m

6,800 m remain after 6 weeks