OFFICIAL HINT: If you have solved all the other challenges up to this point, you should be able to solve 6a. There is nothing new here. Go back over the previous hints (from the other challenges, parts a and b), and work through it logically. You know some cribs, you know how to do frequency analysis, and build from there. More specific clues will be given in time.
OFFICIAL HINT: This is a cipher you did just last week if you solved both parts a and b. Think what your choices are for the first word (length of word will help you). Then look back at the official hints for 5a and 5b. See if you can figure out which of those 2 ciphers is being used here.