The distributive property - a beginner video lesson
In this basic lesson I show several examples of using the distributive property of multiplication over addition, including an area model (rectangles). Lastly we even touch on using the distributive property "backwards", which is the process of factoring algebraic expressions.
I don't use any negative numbers in the expressions as this is meant as a simple, beginner lesson on this topic. Algebra and pre-algebra students will need to study the usage of the distributive property also with negative numbers - but they can start with this video lesson to get a solid understanding of the concept itself. For 6th grade math it's not necessary to learn the distributive property with negative numbers.
In the video, I first work out several simple examples such as simplifying 3(x + 5) and 5(2a + 7). Then, the area model has to do with a two or three-part rectangles. For example, the expression a(2 + b + 3) gives us the area of a rectangle with sides a and 2 + b + 3, but it is also composed of three smaller rectangles with areas 2a, ab, and 3a. It is easier to see in the actual visual model than by reading this. :)
See also
Math Mammoth Grade 7 curriculum (pre-algebra)