First, rewrite this expression as:
#((-y)/6)/(z/9)#
We can now use this rule for dividing fractions:
#(color(red)(a)/color(blue)(b))/(color(green)(c)/color(purple)(d)) = (color(red)(a) xx color(purple)(d))/(color(blue)(b) xx color(green)(c))#
#(color(red)(-y)/color(blue)(6))/(color(green)(z)/color(purple)(9)) = (color(red)(-y) xx color(purple)(9))/(color(blue)(6) xx color(green)(z)) = (color(red)(-y) xx (color(purple)(3 xx 3)))/((color(blue)(3 xx 2)) xx color(green)(z)) = (color(red)(-y) xx (color(purple)(color(black)(cancel(color(purple)(3))) xx 3)))/((color(blue)(color(black)(cancel(color(blue)(3))) xx 2)) xx color(green)(z)) =#
#(-3y)/(2z)#
