The answer is gonna be (4,3) .
if the order mattered it would be 7*6*5 *4*3 = 2520 arrangements
if order didn't matter
7*6*5*4*3*2 divided by (5*4*3*2*2) = 21
I think it is the Associative Property of Addition.
here is a link to the aneser hope it helps
