@yagazie
Okay I'm thinking E23
Reason: The difference between A11 and B17 is 6. The difference between C17 and D5 is 12.
So I'm thinking the difference between the alphanumerics are multiples of 6.
So 6, 12, 18.
E23 minus D5 gives 18.
Placing C17 after B17 might be a ploy to make the riddle harder, idkπ€