Multiple choice question, do your best

Choice B) 
(y+5)+y = 6



So what we do is start with x+y = 6. We can replace the 'x' with (y+5). This replacement is possible because x = y+6; ie x and y+6 are the same. Another term for "replace" is "substitute"

This is how we're able to go from x+y = 6 to (y+5)+y = 6