You insert the (t + 1) in for the x correctly, but didn't carry the "+ 1" on the end with it. That isn't the same "+ 1" as the one for the x. It's completely separate.
Put parenthesis around the x in the function. Then replace that x with the (t + 1), but keep everything else. See what I mean?
(Is that an acceptable answer, or are you also supposed to foil, combine like terms & all that?)
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