Question 673244
Yout teacher gave you a code!  "y = x + 1" means that your teacher added 1 to each letter.  For example, if the secret letter was "A", which is 1 in the alphabet, then "x + 1" would equal 2, or "B".  She woud give you "B" and you would subtract 1 to find A.
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So, each letter your teacher gives you is "y":
y = x + 1
B = A + 1 (for example)
2 = 1 + 1
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In your code, each letter that your teacher gave you is one ahead of what it should be (x + 1).  So, J = I, M = L, etc...