Again, I suggest you familiarize yourself with this chart. Memorization is not necessary, as this will come with time, but you should be able, with pencil and paper, to convert any prefix length or subnet mask back and forth between the two forms.
For example, a prefix length of 19 corresponds to a subnet mask of 255.255.224.0. Why? Write out 19 one bits, pad them with enough zeros to form a thirty two bit subnet mask (32 - 19 = 13 zeros), and then divide them into four groups of eight bits each:
Convert each byte to decimal and you get 255.255.244.0
If you are having difficulty seeing the relationship between these numbers, I again suggest you review the Binary Arithmetic page, as well as the last few pages.