input: Write a python function to count the occurrences of a given element in a list.
  assert count_x([1, 2, 3, 2, 4, 2, 5], 2) == 3
  assert count_x([1, 2, 3, 4, 5], 6) == 0
  assert count_x([1, 1, 1, 1, 1], 1) == 5
output: def count_x(lst, x):
      return lst.count(x)