n = [[1, 2, 3], [4, 5, 6, 7, 8, 9]]
results = 
for numbers in lists:
for a in range (0, len(lists)):
results = results + lists[a]
'for a in range (0, len(lists))'
This says for variable a in the length from zero to the length of your list plus a variable results is equal to itself. I believe you can simplify this code even further and say
results += lists[a] (adds results to lists[a] and makes it equal to itself). But
results = results + lists[a] is... DA DA DA DAAAA! STRING CONCATENATION!!
You may want to more carefully read the instructions:
'Create a function called flatten that takes a single list and concatenates all the sublists that are part of it into a single list'
Here is a link on string concatenation: