input: def find_index(arr: List[int], x: int) -> int:
      """ This function takes an array and a value and returns the index at which
      this value is present in the array. If the value is not found in the array,
      it returns -1.
      
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 20)
      -1
      """
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1