$ P(red | 1 red roll) = P(red & 1 red roll) / P(1 red roll) $
Start by calculating the probibility of each dice being picked times its probibility of rolling 2 reds $ = (1/3)(1/6)(1/6) + (1/3)(2/6)(2/6) + (1/3)(3/6)(3/6) $
then you simply take your answer over the aswer to part a to finish the formula and get your answer.
