Khan Academy
Khan Academy: Geometry: Perimeter and Area of a Non Standard Polygon
Explains how to find the perimeter and area of a non-standard, or irregular polygon. [3:03] CCSS.Math.Content.3.MD.C.7.d Recognize area as additive.
Math Mammoth
Math Mammoth: Area of Rectangles and Rectangular Shapes
Demonstrates how to find the area of a rectangle using unit squares. [9:53] CCSS.Math.Content.3.MD.C.7.a Find the area of a rectangle with whole-number side lengths by tiling it. CCSS.Math.Content.3.MD.C.7.d Recognize area as additive.