Answer
$ 6.5\ cm$
Work Step by Step
The relationship of two rotating pulleys connected by a belt is:
(diameter of A)(rpm of A) = (diameter of B)(rpm of B).
Now we have
$$
21\times 65=x\times 210\Longrightarrow x=\frac{21\times 65 }{210}=6.5\ cm
.$$
