If p is the third proportional to 3, 9, then what is the fourth proportional to 6, p, 4?

18

Third proportion to a, b = $\frac{b\; \times\; b}{a}$

Third proportion to 3, 9 = $\frac{9\; \times\; 9}{3}$ 

                                p = 27

Fourth proportion to a, b, c = $\frac{b\; \times\;c}{a}$

Fourth proportion to 6, p(27), 4 = $\frac{27\; \times\; 4}{6}$  = 18