I think you use a IF function but I am not sure.

I have two large lists of numbers. The lists are in columns A and C with
columns B and D blank. In B, I want to insert an "X" if the number in column
A next to it is somewhere in column C. I tried =IF(A2=C:C,"X","") and
=IF(A2=C2;C1000,"X","") but they do not work. Is there a way I can do this?