First, rewrite the expression as:
#9/2 -: -6 => 9/2 -: (-6)/1 => (9/2)/((-6)/1)#
Now, use this rule for dividing fractions to evaluate the rewritten expression:
#(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)(9)/color(blue)(2))/(color(green)(-6)/color(purple)(1)) =>#
#(color(red)(9) xx color(purple)(1))/(color(blue)(2) xx color(green)(-6)) =>#
#(color(red)(3 xx 3) xx color(purple)(1))/(color(blue)(2) xx color(green)(3 xx -2)) =>#
#(color(red)(color(black)(cancel(color(red)(3))) xx 3) xx color(purple)(1))/(color(blue)(2) xx color(green)(color(black)(cancel(color(green)(3))) xx -2)) =>#
#3/-4 =>#
#-3/4#
