Using DISTINCT AND SUM together SQL question

I've got a large table that I'm trying to write a query for. I don't fully understand the grouping functions, and I'm having some trouble getting what I want from the table. The table contains many columns but I am only interested in 3 of them. Call them ITEM, REV, and QTY. I (simply) want to do a SUM(QTY), but only for distinct rows of ITEM and REV. That is to say,