Hello, @EYC,
It is good to hear that you found this place helpful!
Regarding your second question, since the grader runs the code cells in the sequence presented in the notebook, Cell #11 means exactly the 11th code cell counting from the top.
My 11th cell has no x at all, so you might want to check if you had added any new cell or modified any provided cell. If there was any modification that you were not sure how to revert, the best way would be to follow these instructions to get a new copy, then move your work there and submit!
Let us know how it goes!
Cheers,
Raymond