In a Fibonacci Sequence, the following applies:
1,1,2,3,5,8 etc.
Where the following term is a sum of the 2 previous terms.
Relate this to the terms in your question:
1, 1, 2, 5, 8
Goes to:
Y-x, x, y, x+y, x + 2y
Since to get y you add the 2 previous terms
y-x + x = y
y + x = y + x
x + y + y = x + 2y
Hope this helps you :)
You used multiplication of 2 terms which is where you went wrong.