I have a column of data, several hundred lines long, that has 10 digits in
each cell (1234567890) and I want to use the formula
=LEFT(A1,3)&"-"&MID(A1,4,3)&"-"&MID(A1,7,3) to make the data look like
123-456-789. I need to copy and paste the formula to a set of helper cells
but don't know how to copy & paste the formula.

Any help would be greatly appreciated.

Thanks