FP2 series question

Any help on the following would be greatly received.. (I know it's late!)

(JAN 2010 AQA)

Q 7. b)



I don't understand how they go from the 1st line to the 2nd line in the mark scheme. I of course get the 'n' part, but not how they automatically get 3(2^n -1)

Thanks anyone that can help at this ungodly hour!

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