Well, that is the Commutative Law of algebra. You actually knew that, although you may not have actually thought about it. Here we have 4x3 trees, which is the same as 3x4 trees, which is the same as 12 trees. It doesn't matter whether you count the trees or do the multiplication short cut. You already knew that multiplication was Commutative.
Subtraction is the inverse of addition and division is the inverse of multiplication.
You probably knew that. That was algebra, mathematics using unknowns. You were doing algebra whenever you subtracted, same with division. You also knew all of the other basic laws of algebra. You just didn't know their names.