HELP: Scala sequence function

[yamileon]

so im doing this scala sequence problem and cant figure it out at all 

im able to get the code working but the math not so much 

 

edit: even how to read the sequence An =(-1).... would help 

 

Question in image 

 

def series(n:Double):Double={
  def tail(acc:Double,n:Double):Double={
    if(n < 1)acc
    else {
      tail( acc + ( math.pow(-1,n) * ( (5 + (5 + (n-1) ) ) + math.pow(-1,n-1) ) * (3 + (3 + (n - 2)) ) ), n - 1 )
    }
  }
  tail(0,n)
}

series(3)

what i have so far