Simplify the following: (r+s--s+t)

R+s-t-(r-s+t)

=2s-t is the answer

People think this answer would be just =2s but I have verified with an online calculator and the answer that textbooks look for regarding this is 2s-t

You would assume you would go r-r and thats 0 so no r's

you would say s-(-s) makes it plus and theres 2s

and you would say -t -(+t) is -t 

that's why the answer is 2s-t

At first I also did t-(+t) and thought it would be no t's but the answer is in fact 2s-t 

Are there any statements?