notice how in both diagrams, there are 4 triangles with lengths a, b, c, but in each diagram the triangles are just rotated and positioned differently. if there are the same amount of triangles in both diagrams, and the have equal lengths then the area of the remaining figure, should both match up.
basically the area of the big square in Step 2, is equal to both of the shaded squares, combined area, in Step 1.
