draw a square with an equilateral triangle (the side must be as big as the side of the cube) joined by its side on all sides.
It will look like a 2D equilateral triangle if it is a triangular based pyramid which is called a tetrahedron.
a larger equilateral triangle
A triangle and a square
4 triangle surfaces
A square-based pyramid net consists of a square base with four isosceles triangle faces extending from each side of the square. The triangles meet at a single point at the top of the pyramid. When the net is folded and assembled, it forms a three-dimensional square-based pyramid shape.