The pattern is plus 7
6 + 7 = 13
13 + 7 = 20
20 + 7 = 27
This means to find the next term you must add 7 to 27
27 + 7 = 34
To find the term after 34 add seven to that as well
34 + 7 = 41
so...
6, 13, 20, 27, 34, 41
Hope this helped!
~Just a girl in love with Shawn Mendes