Respuesta :

morgeron
look for patterns between the numbers first and see where it goes...

3,    -6,   12,    4,   20 ((distance between 3 and - 6 is -9)
  -9    +18    -8  +16  (let's continue the pattern )

20,    13,       27,     21,      33.......
     -7      +14      -6      +12