Can't get teachers name

# 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(string):
    return len(string)
def num_courses(string):
    return sum(len(k) for k in string.values())
def courses(string):
    courses_list = []
    for items in string.values():
        courses_list.extend(items)
    return courses_list
def most_courses(string):
    sort = []
    sort = sorted(string.items(), key = lambda i : len(i[1]))
    return (sort[-1])