Why i cant use this method? enter to understand my question

def courses(num_courses):
    list_courses = []
    for x in num_courses.values():
        list_courses.append(x)
    return list_courses

def courses(num_courses):
    list_courses = []
    for x in num_courses.values():
        list_courses.extend(x)
    return list_courses



thanks in advance ;D

```teachers.py
# The dictionary will look something like:
# {'Andrew Chalkley': ['jQuery Basics', 'Node.js Basics'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Each key will be a Teacher and the value will be a list of courses.
#
# Your code goes below here.

def num_teachers(teachers):
    return len(teachers)

def num_courses(courses):
    number = []
    for course in courses.values():
        number += course
    return len(number)

def courses(num_courses):
    list_courses = []
    for x in num_courses.values():
        list_courses.extend(x)
    return list_courses