One man will do 1 / (3 * 17) = 1 / 51 of the work in one day.
One boy will do 1 / (8 * 17) = 1 / 136 of the work in one day.
2 men and 6 boys will do:
(2 / 51) + (6 / 136) = 1/12 of the work in one day.
Hence, time taken to do all the work will be 1 / (1/12) = 12 days