how can i get the slope function to ignore missing data?

[Delmar]

I have a spreadsheet that needs to calculate the slope of a line. The x
values are in column a, the y values in column d. The slope function in
excel 2003 works just fine for this purpose until it encounters missing data.
The slope function returns #N/A. I have tried entering #N/A for the missing
data but the result is the same. How can I get it to simply ignore my
missing data?