Click and hold the Rectangle tool until the sub tools pop up.
Select the Polygon tool
Click just once on the artboard and an options window will appear.
Enter the number of sides you want and the general size (Radius) and click OK.
Select the Selection Tool (The Black Arrow) and hold down the Shift and Option/Alt Key. Click-drag the triangle downward. When you let go, the original triangle with be duplicated in the new position.
Option/Alt while dragging tells Illustrator to duplicate the object. The shift key keeps the dragging constrained to 90° angles. In this case, straight downward.
You should then have 2 overlapping triangles.
Select both triangles and click the Minus Front button on the Pathfinder Panel (Window > Pathfinder)
After clicking the button you are left with the basic shape which can then be further edited.