input: def find_index(arr: list, x: int) -> int:
      """Given an array and a value, returns the index at which the value is present in the array. If the value is not present, returns -1.
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 15)
      3
      >>> find_index([2, 5, 9, 15], 7)
      -1
      """
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1