Date Of Last Transaction

I'm dealing with 2 spread sheets. The first, "Transactions", lists transactions by date, but they are grouped by series. So the 7 series dates are ascending but when you continue to the 8 series it starts at the first date of transaction and ascend to the latest. The second spread sheet, "Sheet1" is an inventory and lists date of last transaction, part #, Part name, sorted ascending by part #. I need a function that will reference the Part # in sheet1 and yeild the date of the last transaction made with that part #. I have not been able to figure it out.