If point a, having coordinates (9,p) and point b, having coordinates (p,7), lie on a line having a slope of -1/3, what is the value of p?

The slope of the line =  y2 - y1 / x2 - x1

                                = (7-p)/ (p - 9)

and this equal -1/3

cross multiplying  -1(p -9) = 3(7 - p)
                             9 - p = 21 - 3p
                              -12 = -2p
                               p = 6

There is 240 red cars and there is 400 blue cars because 240/3=80 and 80*5=400