[tex]\it \mathcal{A}_{\ell} =\pi RG=15\pi|_{:\pi} \Rightarrow RG = 15\ \ \ (1)[/tex]
[tex]\it \mathcal{A}_t = \pi R(R+G) =24\pi |_{:\pi} \Rightarrow R(R+G) =24 \Rightarrow R^2+RG =24 \ \ (2) [/tex]
[tex]\it (1), (2) \Rightarrow R^2+15 = 24|_{-15} \Rightarrow R^2 = 9 \Rightarrow R = 3 cm[/tex]