The first sequence, it is increase +1 for every number, because 2-3 is 1, 3 - 5 is 2, and 5 - 8 is 3. So, lets add 4 to 8, and we will know the next number of the sequence.
4 + 8 = 12
So, the next number in the first sequence is:
1, 1, 2, 3, 5, 8, 12
Second Sequence:
The second equation is doing exactly the same thing (it increases by 1 every number it goes) just this one is decreasing.
Proof: 26 - 24 is 2, 24 - 21 is 3, and 21 - 17 is 4.
Lets subtract 5 to 17 to know the next number.
17 - 5 = 12
So our next number is:
26, 24, 21, 17, 12