Copypasting detected:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    Test.assert_equals(longest_slide_down([
NameError: name 'longest_slide_down' is not defined

This is a duplicate of https://www.codewars.com/kata/pyramid-slide-down

In general, copy an coding problem from another entry-level coding site without attribution is a bad idea, because people have already taken all these low-hanging fruits, and they're often not very original. Not to mention that your solution (aka the reference solution) is wrong.