How is the U.S. government able know where hacks originate from?

There is a well-known technique in SIGINT called traffic analysis, which lets us know a lot about where stuff came from without actually seeing the content, though we can infer that it must be true.

For example, at one time, the entrance exam for the NSA included a traffic log of communications between a set of 4 islands without identifying who was on what island and what they where doing.  

Your job was to identify which islands were at war with other islands, determining the military and political headquarters of each "nation", where the troops were, where various battles had taken place, etc.  

This was based 100% on who called whom, for how long and no other data. The NSA and other intelligence agencies have been at this for a very long time, and they know how to--literally--pull information out of thin air, knowing that it is correct.
What is the best way to learn Excel online?

For advancing in functions & formulas, I recommend Someka's Excel Formulas Training - Free Video Course from Beginner to Advanced which is quite engaging and easy to follow as it goes one by one on each function.

What is the full form of SIR?

Hello,The word Sir is a respectful form of address for a man and a formal title used in the United Kingdom for knights and baronets. (This is the most commonly use of the word)Here are some other full forms that might help

Will we, in next 300 years, see Star Trek type space travel?

It's possible, though we are pushing up against the fundamental laws of physics as we currently understand them. Technological advancement is progressing exponentially, we've come further in the last 100 years than we did in the 1000 before that. If you go back 300 years we are in 1718, just after the invention of the