Math teacher here. I'm assuming bigger = volume. A ball that's twice as big is not also twice the surface area.

You have to take the volume of Earth and multiply that by 2.4 first, find the radius, then take the surface area of your new planet (using S/V = 3/R) and multiply that by 0.31.

Earth's current volume = 259,875,159,532 mi³

New Earth volume = 623,700,382,877 mi³

New Earth radius = 5300.24323572 mi

New Earth total surface area = 353,021,751 mi²

31% = 109,436,742 mi²

Which is also what RonaldRayGun got but in a different way, and he showed his work. Full credit.

This is the info I asked for earlier: "Did you mean 2.4x volume, surface area, etc?" I solved for volume, but you meant to solve for diameter/radius. Kind of hard to answer the question without the proper info.

But I'm glad you got your answer. Pretty cool about Keplar-22b. I remember hearing about this planet but completely forgot to read about it. This is good stuff, thanks!