Answer: Choice B)
(y+5)+y = 6
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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