def find_sum_pair(val, key):

  for index in range(len(val)):
     for ix in range(index + 1, len(val)):
        if val[index] + val[ix] == key:
           return True
  return False

my_list = [[71, 5, 21, 6], [34, 21, 2, 71], [21, 2, 34, 5], [6, 9, 21, 42]]

print("列表是：")
print(my_list)

my_key = 76
print("关键字是")
print(my_key)

my_result = [element for element in my_list if find_sum_pair(element, my_key)]

print("结果列表是：")
print(my_result)

列表是：
[[71, 5, 21, 6], [34, 21, 2, 71], [21, 2, 34, 5], [6, 9, 21, 42]]
关键字是
76
结果列表是：
[[71, 5, 21, 6]]