With only two numbers it is difficult to find a pattern but I would imagine that this is a simple one since those are the only two numbers provided. If you do 21-13=8 you see that the difference is 8 so therefore, you would add 8 to each continuing number of the sequence.
The pattern would be 13, 21, 29, 37, 45, 53, 61, 69
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24, they are going up in 3's, 13, 16, 19 and 18, 21 so next number is 24.
24
35
21
43