What property is in (6+3)+21=6+(3+21)

This is called the Associative Property of Addition.

In short, associative property of addition states that the quantity of an additive expression is going to be the same no matter how the expression is grouped. In this case, it doesn't matter whether 6+3 is added first or 3+21 is added first because they will end up with the same quantity.

hello :

by applying the distributive property :

6 × 4.3 = 6 × ( 4 + 0.3 )

            =   (6 ×   4) + (6 ×   0.3)

            = 24 + 1.8

6 × 4.3 = 25.8