Count calcuation

Hi folks.

I am trying to use this count function to calculate number of lines within each PO but not getting the desired result. What is the best approach to this?

the fields are Vendor, PO, PO Line# ... using these I need to get the count of PO Line# at Vendor level.

sample data:

Vendor     PO     PO Line#

111           100     1

111           100     2

111           100     3

111           101     1

111            101     2

112            100     1

113            100     1

113            100     2

result should look like this:

Vendor     Count

111           5

112           1

113           2

Thanks for your help.