Calculate a radius for Pluto using Sun, Pluto brightness and albedo.

Spectroscopic observations suggest that Pluto is covered with icy frost and thus has a high albedo (0.5). The brightness of Pluto at opposition (38 AU from the Earth) is 2 x 10^-17 as bright as the Sun (1 AU from the Earth. From these two observations, calculate a radius of Pluto.