An added note: I have discovered that the sooner you get the other letters (in this case a_y_a_ and s), the sooner the missing letters (k in this case) will show up. So, in addition to getting long words, get the a, y, a, and s ASAP!

@Kathleen - By now you gave up or figured it out, but for the benefit of others: The red "4's" in the corners mean you can only use that letter in a word that is 4-or-more letters long.
There are other numbers in the corners that are "countdown timers". They will end your game sooner if you don't destroy them before they hit zero.

Green tiles will change to red on every other turn (you can't make words from red tiles). Regular white tiles are free to use pretty much all the time.

The word is kayaks. They don't give you any K's right away. Best strategy is longest possible words to get rid of the stops and get more letters to drop ASAP. Hope that helps.