Respuesta :

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Answer:

15

Step-by-step explanation:

If this is an arithmetic progression, then there is a common difference.

-3-18 = -21

-21/3 = -7

18 - 7 = x

x = 11

11 - 7 = y

y = 4

4 - 7 = -3

x + y

11 + 4 = 15

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