You can put this solution on YOUR website!
The unfortunate part of sequences and series is that there is usually more than one answer. However, if you know what to look for, then it's not so bad.
Try breaking up the sequence to get:
Notice how the red terms are decreasing by 2 while the blue terms are increasing by 1. The red and blue terms alternate in sets of 2. In other words, there are two subsequences (a smaller sequence) alternating every two terms. So up next is a blue term.
To get the next term, add 1 to the last blue term to get 18+1=19. After that, subtract 2 from the last red term to get 6-2=4.
So the next two terms are 19 and 4. The obvious problem with this solution is that you have to know the answer to have an idea of what to do here. However, it is possible to just stumble onto it.