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Explanation:
4y is the middle value, or second value.
This means 4y-1 is the first value since it is immediately before 4y
Also, we know that 4y+1 is the value just after 4y
The order of the terms is: 4y-1, 4y, 4y+1
Add up those three expressions and simplify
(4y-1) + (4y) + (4y+1)
4y-1 + 4y + 4y+1
(4y+4y+4y) + (-1+1)
12y+0
12y
The three terms combine to 12y.
