How to show month and year in one column for report

Hello. I created a report to track Opportunities created by Sales Rep.  As part of the compensation plan for our Sales Reps, we need to track how many opportunities have been created by month and year. I put together a report based on the "created date". Has to be the Created Date - when the Reps created the opportunity.

I want the "Created Date" field on the report to include the month and year. I see in the report builder how you can change the formula on this field to do this. However, when I combine the formula to include both month and year I get all kinds of syntax errors.