Associative property of (21)(7)(3x) ?40

Ehh, now it's letting me answer.

So the associative property is basically as long as it's addition or multiplication,  you can move around the groupings and it will still be the same.

For example, you can say:(21 * 7) (3x)  instead of (21)(7)(3x)

And it would be equal to the old version.

(To answer your other question, the commutative property is you can move around all the parts of the equation as long as it's addition or multiplication.)

So basically, it would be the first one you sent me.

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.