Two circles touch each other externally. The radius of the first circle with centre O is 6 cm. The radius of the second circle with centre P is 3 cm. Find the length of their common tangent AB.

A

$$3\sqrt{2}$$ cm

B

$$4\sqrt{2}$$ cm

C

$$6\sqrt{3}$$ cm

D

$$6\sqrt{2}$$ cm