OK, so we're doing some Seminole piecing.
What you want are called "setting triangles," which should be cut as QST's so you don't have any bias on the outside edge. You can google or search this board for more detailed explanations.
The formula to figure out the size of the square needed is the finished size of your desired block x 1.41.
Round up to the nearest 1/4", and then cut the square diagonally twice. (I actually round up more because I prefer my blocks to float.)
2" x 1.41 = 2.82 Cut the squares 3".
Can I suggest starching the fabric well before cutting as there will be exposed bias edges? And you really don't need them to stretch while trying to put this together.