Excel 2007 : Help with AVERAGE formula based on values in other cells

Hello, I'm stuck with something I am trying to calculate in a spreadsheet and hoping someone can help.

I want to work out the average of a set of data in column AM. However, I only want to include the cells in AM when the corresponding values in column D and column AN both equal the word "Yes". Please can someone explain how I would write this formula?

I hope this makes sense

Many Thanks