Answer:
C
Step-by-step explanation:
5ax²-20x³+2a-8x
=5 x²(a-4x)+2(a-4x)
=(a-4x)(5x²+2)
Set the 2x - 1 equal to 9 then work backwards. 2x - 1 = 9 Nine minus one is eight divided by two is four. X = 4
This one is tricky....but I'll go for D cause your multiplying a number towards 5
He must sell $200 worth of electronics.
200 x .5( 5%) = 100.
Not addition, since

and 0 doesn't belong to the set.
Not subtraction, since

.
It is closed under multiplication. The set appears to contain all non-zero multiples of 3.
Not division, since dividing some of the numbers results in rationals which do not belong to the set.
What other operations are you supposed to consider?