input: def find_index(arr: List[int], x: int) -> int:
      """Given an array arr and an integer x, return the index of x in arr.
      If x is not present, return -1.
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([1, 2, 3, 4], 6)
      -1
      """
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1