Question

how quickly must a 54.7 g tennis ball travel i order to have a de Broglie wavelength that is equal to a photon of green light?

Answer 1

wavelength = h/mv
Substitute the wavelength of the green light you have in mind (in meters), use mass in kg and v will be in m/s.

Answer 2

nearest star distance of 4.3 light years proxima centauri is the nearest star to our solar system. what is the distance to proxima centauri kilometers? the speed of light in space is 2.998 x10^8 m/s.