Too be honest I did some research on it it says “Every surface on earth absorbs and reflects energy at varying degrees, based on its color and texture. Dark-colored objects absorb more visible radiation; light-colored objects reflect more visible radiation. Shiny or smooth objects reflect more, while dull or rough objects absorb more.”
Their is 18 dogs in total because you would do 6x+x=21 and when you solve it x= 3. To figure how many dogs their is you would just plug in 3 for x, which would look like 6(3).