Why is there no middle term in the product(a-b)(a+b)?

The function a represents the cost of manufacturing product a, in hundreds of dollars, and the function b represents the cost of manufacturing product b, in hundreds of dollars. a(t) = 5t + 2 b(t) = 7t - 2t +4 find the expression that describes the total cost of manufacturing both products, a(t) + b(t). a. 7t^2 - 7t + 2 b. 7t^2 + 3 +6 c. 7t^2 + 7t - 6 d. 7t^2 - 3t + 6

What is not true about kl and mn? what is mn? (show )

Sections of prefabricated woods are each 5 1/3 feet long, how long are 7 1/2 sections placed end to end?