An urn contains 9 white, 9 red, 9 blue and 9 green balls. 4 balls are drawn randomly without replacement and a fifth bonus ball is drawn from the same urn.
a) What is the probability of drawing 3 red balls and a non-red bonus ball?
b) What is the probability of drawing 2 black balls and 1 black bonus ball?
I managed to use a general formula for drawing 5 balls but I cannot modify it for the required solution. Please refer to the attached images for my progress.