1 kilometer = 6.68 × [tex]10^{-9}[/tex] (<<<AU is in scientific notation here)
To convert 300 billion kilometer (3,000,000,000 km) into AU you must multiply the kilometer by the equivalent of 1 km in AU (6.68 × [tex]10^{-9}[/tex])
3,000,000,000 × (6.68 × [tex]10^{-9}[/tex]) = 20.04 AU
Hope this helped!