I know that $\displaystyle A=\pi (R+r)\sqrt{(R-r)^2+h^2}$

Question: Two mathematically similar frustums have heights of 20 and 30cm. The surface area of the smaller frustum is 450cm^2.

Calculate the surfce area of the larger frustum

My workings:

$\displaystyle 450=(\pi R+r)^2\cdot (R-r)^2+400$

$\displaystyle 50=(\pi (R+r))^2(R-r)^2$

Now I'm lost. Help please