Well, forst you have to do the opposite to find what y equals. You have to start on the side with the variable. The opposite off adding is subtaction. 
-y+56=167
 -56. 
-y+56=167
 -56
What you do to one side you must do to the other, you mark out 56 and take 167-56. 
That equals 111. 
111=y
I hope this explains why! :)