Hello there,

Based on your query this is how you have to find the solid angle,

The given diameter is d = 3474km

therefore the radius r = d/2 = 3474/2 = 1737km

Area of moon's projection = pi*r^2 (pi r sqaure) = pi*1737*1737 km*2

Now, Solid angle = area/ (distance)*2

= (pi* 1737* 1737)/(3.84*10^-5)*2  given distance is 3.84*10^-8 but converting it into km you get 3.84*10^-5

you get 6.438*10^-9 steradians.

The SI unit of solid angle is seradians

Hope this answer helps.

Thank You!