Splitting the picture in half horizontally, it is mirrored - meaning there is the same number of triangles in the top half as in the bottom. To make it easier, puzzlers can count one set and ...

Image caption, STEP 3 - SQUARE IN MIDDLE: Arrange the triangles on the square so there's one in each corner. Slide the bottom left triangle up towards the right until it meets the triangle top right.

A triangle is a polygon. It is a closed 3-sided shape made up of straight lines ... The square drawn in the bottom right angle shows us that one of the angles is a right angle.