Reselect the tree black rectangles made in the last three steps and Group them (Control-G). Make sure that your new group is selected and go to Effect > Distort & Transform > Transform. Check the Reflect Y box, enter "1" in that Copies box, drag the Move-Vertical slider at 21px and click OK. In the end things should look like in the second image. Move to the Layers panel, double-click on the group made in this step and rename it "drumShading".