Mn = 2ab, so 3mn = 6ab. this is an illustration of which property?

In general: 

For any 3 numbers a, b, and c≠0, 

If a=b then a.c=b.c (a times c = b times c)

This is called the "multiplication property"

In or case, a=|MN|, b=|2AB|, c=3

MN=2AB so 3.MN=3.2ab, so 3MN=6AB

Multiplication property.