3, -6, 12, 4, 20, ? what is the number after?

Sorry I'm not good at this!

 

I wish I could help!

 

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~Hannah🌹🌹🌹

3,   -6,   12,   4,   20

  -9   +18   -8   +16

you can see the pattern from here.

so, 20 - 7 = 13