You'll like draw a perfect square or rectangle by drawing congruent angles to form the box, and then congruent triangles (equilateral ... both angles on the same side must equal 180 degrees ...

Shapes I and L are congruent triangles. When I is rotated 90° anti-clockwise it has the same orientation as L. The sizes of angles must be ... ÷ BC = 6 ÷ 3 = 2. The sides DE and DF are three ...