Question

(2×4)×7=2×(7×4) what property does this demonstrate

Answer 1

Associative property of multiplication  

The property states: 
a*(b*c)=(a*b)*c
Basically says no matter how the numbers are grouped you will get the same answer.  

Answer 2

Well,

As we can see, the only difference is that the parentheses have moved.

This is an example of the associative property.  It is specifically of multiplication, because products are used in this case.

Just as a test, let's see whether they are really equal.

Following PEMDAS, we get:
(2*4)7 = 2(7*4)
(8)7 = 2(28)
56 = 56

They are equivalent.