input: 
  from typing import List
  
  def find_index(arr: List[int], x: int) -> int:
      """
      Find the index of an element in the array `arr` that matches the value `x`.
      If the element is not found in the array, return -1.
      
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 10)
      -1
      """
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
  
  # Example input and output
  arr = [2, 5, 9, 15]
  x = 5
  print(find_index(arr, x))  # Output: 1
  
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1