How do you get terms, coefficients and constants from an algebraic expression

    choose the correct definition for  extremo.  a.  end      b.  extra      c.  extract      d.  eventual 

There are three bags: a (contains 2 white and 4 red balls), b (8 white, 4 red) and c (1 white 3 red). you select one ball at random from each bag, observe that exactly two are white, but forget which ball came from which bag. what is the probability that you selected a white ball from bag a?

Extra points! find the missing dimension! need asap