The answer is 0. Why?
a2x is the just: a * 2 * x (a multiplied by 2 and x). Thus, you can arrange a2x as '2ax' or 'ax2' and it would still mean the same thing. Looking at the equation "a2x - ax2". You can rearrange the "a2x" to "ax2". What is the "ax2 - ax2"?
Another thing to keep in mind:
If a = 2, x= 3
Replace those numbers into the equation: a2x - ax2. It check out.
(2*2*3) - (2*3*2)
12 - 12 = 0
If you're still confused, ask.