This question was marked a duplicate: Tried to install wireless driver and now terminal and update manager no longer work either
However, the link given, Installing Broadcom Wireless Drivers contains several answers that are just plain incorrect. How is this brand new user supposed to wade through 38 answers and decide which one or ones are correct and which are just plain wrong? He already tried bcmwl-kernel-source as suggested in the alleged answer and it didn't work! It didn't work because it is just wrong for his 14e4:4312 device.
Please help me understand, if it is indeed possible to understand Broadcom wireless. Even better, please un-duplicate his question and let me answer him.