Will the nexplanon definitely work on the seventh day ?

Depends on time : of cycle that it was placed, to be sure use condoms for the first 4 weeks, and also to decrease viral infections.

Nexplanon: Nexplanon is a progestin contraceptive, and if properly inserted, is about 97-99 % effective at the 7th day. Remember that ovulation is frequently delayed on Nexplanon, so your periods may not be regular. If pregnancy symptoms occur, get a serum HCG test to be certain about pregnancy.