First, simplify given expression:
color(white)=(27r^5)/(7s)*(28rs^3)/(9r^3s^2)
=(27r^5)/(7s)*(28color(red)cancel(color(black)r)s^3)/(9 r^stackrel2color(red)cancel(color(black)3)s^2)
=(27r^5)/(7s)*(28s^color(red)cancel(color(black)3))/(9r^2color(red)cancel(color(black)(s^2))
=(27r^5)/(7s)*(28s)/(9r^2)
Next, multiply the numerators and denominators:
color(white)=(27r^5)/(7s)*(28s)/(9r^2)
=(27r^5*28s)/(7s*9r^2)
=(756r^5s)/(63r^2s)
=(stackrel12 color(red)cancel(color(black)756)r^5s)/(color(red)cancel(color(black)63)r^2s)
=(12r^(stackrel3 color(red)cancel(color(black)5))s)/(color(red)cancel(color(black)(r^2))s)
=(12r^3color(red)cancel(color(black)s))/color(red)cancel(color(black)s)
=12r^3
This is as simplified as it gets.