I'm guessing from the fact that you define x1 but don't use it that the x variable may not exist. You may have been thinking / hoping that MATLAB would use x1, x2, etc. when it saw x(1), x(2), etc. but it does not.
The fact that you're using x(k+1) (not x(k-1)) in your while condition also smells a bit strange. You may be walking off the end of the x array.
But if correcting these does not resolve the problem, please state more clearly the specific difficulty you're experiencing.
Do you receive an error message (text displayed in red)? If so, tell us on which line the error occurs and show us the full and exact text of the error, all the text displayed in red exactly as it's displayed in the Command Window.
Do you receive a warning (text displayed in orange)? If so, do the same as for an error but show us all the text displayed in orange.
Do you get a different answer than you expected? If so, show us the answer you received and the answer you expected and explain why you expected what you expected.
Does something else happen? If so, what?