Why does the Torah see fit to state "Sheish Shanim Tizra Sadecha ... "?
Ramban #1, Moshav Zekenim #1: It is the way the Torah normally speaks. 1
Ramban #2, Moshav Zekenim #2 (citing Yerushalmi in Kil'ayim): It comes to teach us that someone who works his land in the Shemitah transgresses an Asei 2 (as well as the Lav in the following Pasuk.
Ramban #3 (citing the Mechilta in Mishpatim) and Seforno: When Yisrael do the will of Hashem they experience one Shemitah (per seven years), as the Torah writes "Sheish Shanim Tizra es Artzecha ... "; but when they don't, they have to keep four Shemitos - by leaving the land fallow in the first, third and fifth years, 3 followed by the Shemitah in the seventh.
Moshav Zekenim: It is written after "Shabbos la'Shem" to teach that if you observe Shemitah properly, you will sow your field, i.e. you will not need to sell it; if not, you will need to sell it, and also your house and daughter (Kidushin 20a).
What are the implications of "v'Asafta es Tevu'asah"?