From  the earlier grades students knew the formulas for calculating the areas of squares and rectangles. 

One of the groups proved that we can easily transform square for example into some other geometric figure with the same surface. 

Second group transformed a rectangle into a parallelogram and came up with the formulas for the surface of parallelogram.

Third group transformed a  rectangles into a rhombus and received both formulas for calculating a surface of rhombus.