I am working in a large spreadsheet and need to be able to subtract to
columns without adding any new columns or using subtotals. If I have words
and numbers in the columns, ex. "none", is there a function that will
eliminate the "value" error message when I subtract (ex. 9-none = value, but
i want none or any text to be viewed as zero)?

Ex. of how it is currently (not) working
none - 9 = value (wrong)
12 - 9 = 3 (right)

Ex. of how I would like for it to work
none - 9 = -9 (right)
12 - 9 = 3 (right)