How can the area of the non- rectangle be found? - Answers

It depends on what the shape is. There are relatively simple formulae for triangles, circles, parallelograms, regular polygons; and more complicated formulae for ellipses, irregular polygons and other shapes. If the shape can be described algebraically, then integration may provide an answer. Then there are non-analytical methods for very irregular shapes. You can get an estimate by mapping the shape onto a grid and counting the number of squares covered and part covered. Or you can cut out an identical shape from a uniform lamina and compare its mass to the mass per unit area.