Como calcular 3-6 em binário de forma que o resultado (em binário) seja -3?

Me with this program in c++ ! computers represent color by combining sub-colors red, green, and blue (rgb). each sub-color's value can range from 0 to 255. thus (255, 0, 0) is bright red. (130, 0, 130) is a medium purple. (0, 0, 0) is black, (255, 255, 255) is white, and (40, 40, 40) is a dark gray. (130, 50, 130) is a faded purple, due to the (50, 50, 50) gray part. (in other word, equal amounts of red, green, blue yield gray).given values for red, green, and blue, remove the gray part. ex: if the input is 130 50 130, the output is: 80 0 80. thus, find the smallest value, and then subtract it from all three values, thus removing the gray.