the distributive property states that a(bx+c) is equal to abx+ac. in this expression, a is 7, b is 3, and c is negative 4. ab is 7*3, which is 21, so abx is 21x. ac is 7*-4, which is -28. combining these two results in 21x-28.
because if 5students brought in a set of 8 box's with ten in it you get 80 times 5 you get 400 then you add the ten students that brought in a set of 40 each which is another 400 so400 +400= 800
