hello, can anybody help me to setup this problem?

a game begins by choosing between dice A and B is some manner such that the probability that A is selected is 'p'. The die thus selected is then tossed until a white face appears, at which time the game is conluded. Die a has 4 red and 2 white faces. Die B has 2 red and 4 white faces. After playing this game a great many times, it is observed that the probability that a game is concluded in exactly three tosses of the selected die is 7/81. Determine the value of p.

cheers

ronaldo