combine the contents of the two columns into a new column

That really depends on what you want, do you want to add values (C1 = A1+B1), Add two bits of text or text and numbers together (C1 = A1&B1) or even with a comma inbetween (C1 = A1&", "&B1). It all depends on what you want, with your current explanation we have to guess.

To best describe or illustrate your problem you would be better off attaching a dummy workbook. The workbook should contain the same structure and some dummy data of the same type as the type you have in your real workbook - so, if a cell contains numbers & letters in this format abc-123 then that should be reflected in the dummy workbook.

To attach a file to your post,
click advanced (next to quick post),
scroll down until you see "manage file",
click that and select "add files" (top right corner).
click "select files" find your file, click "open" click "upload" click 'done" bottom right. click "submit reply"

Once the upload is completed the file name will appear below the input boxes in this window.
You can then close the window to return to the new post screen.

Ferdy