input: def find_index(arr, x):
      """ Find the index of the given value in the array.
      If the value is present in the array, return the index of the value.
      If the value is not present in the array, return -1.
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 7)
      -1
      """
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1