The first expression is equivalent to B
The second expression is equivalent to A
The third expression is equivalent to C
You are basically adding like terms. For example for the first expression, it is 7x^2 + 2x^2, which equals 9x^2. Remember since you're adding, do not add the exponents.
If you pick only positive numbers, the absolute value will have no effect, and the original set and the set of the absolute value will be exactly the same:

is still

Associative property works in addition and multiplication.
Associative property in Addition: (a + b)+ c = a + (b + c)
Associative property in Multiplication: (a x b) x c = a x (b x c)
Associative property in Subtraction: (a - b) - c is not equal to a - (b - c)
Associative property in Division: (a divided by b) divided by c is not equal to a divided by (b divided by c).
Thus, associative property is not true for all integers.
He put in the value of x into y
and the value of y in x
if that confuse you here this is what it will look like
(x,y)->(y,x)