Find number deleted from array

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I am quite new to ruby so bear with me, I have two arrays and we are meant to find the number which is missing.

The starting array sequence is [1,2,3,4,5,6,7,8,9] The mixed array with one deleted number is 5 [3,2,4,6,7,8,1,9]

My idea is to add up each array and see which number is missing- I started to produce a method (I know it is shabby but I am learning)

def find_deleted_number(arr, mixed)
  arr = [1,2,3,4,5,6,7,8,9]
  mixed = [3,2,4,6,7,8,1,9]
  y = arr.inject(0){|sum,x| sum + x }
  x = mixed_arr.inject(0){|total, y| total + y}
  return y - x
end

Could anyone guide me to what I am doing wrong here?

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