input: Write a function to count the number of occurrences of an element x in a list y. If the element is not found in the list, return 0. If the list is empty, return 0.
  assert count_x(2, [3, 5, 2, 7, 4, 3, 2])==2
  assert count_x(3, [3, 5, 2, 7, 4, 3, 2])==2
  assert count_x(7, [3, 5, 2, 7, 4, 3, 2])==1
  assert count_x(8, [3, 5, 2, 7, 4, 3, 2])==0
  assert count_x(5, [])==0
output: def count_x(x, y):
      return y.count(x)