my_list = [[2, 4, 6, 7], [2, 4, 8], [45], [1, 3, 5, 6], [8, 2, 1]]

print("The list is :")
print(my_list)

my_key = 3
print("The key is")
print(my_key)
my_result = sorted(my_list, key=lambda row: sum(row), reverse=True)[:my_key]

print("The resultant list is :")
print(my_result)

The list is :
[[2, 4, 6, 7], [2, 4, 8], [45], [1, 3, 5, 6], [8, 2, 1]]
The key is
3
The resultant list is :
[[45], [2, 4, 6, 7], [1, 3, 5, 6]]