Find the length of their direct common tangent.

Length of direct common tangent = √(d² − (r₁−r₂)²), where d is the distance between centres. Here d=13, r₁−r₂ = 9−4 = 5. Tangent length = √(169−25) = √144 = 12 cm. (Note: for the transverse/indirect tangent the formula uses (r₁+r₂) instead.)