Depends:
It could be -366 - if you fit a quintic polynomial to the 6 points.
or
It could be 212 - if the sequence is +1, *4, +1, *4 etc
11
11
13
13 if you refer to a number line. If you subract you will get -1-12 = -13
12 and 13
What would be the next number in this series 15 12 13 10 11 8?
20
The sequence alternates between a number and that number plus one. After 2 comes 3, after 3 comes 12, after 12 comes 13, after 13 comes 52, and after 52 comes 53. Following this pattern, the next number after 53 would be 212, as it follows 52 multiplied by 4 (the previous non-incrementing number). Therefore, the next number in the sequence is 212.
13 is one possibility.
240
8
The next number is 13... you're adding the previous two numbers together to produce the next in line.