Rewrite the expression as:
#3/4 -: 6 => 3/4 -: 6/1 => (3/4)/(6/1)#
Now use this rule for dividing fractions to complete the simplification:
#(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)(3)/color(blue)(4))/(color(green)(6)/color(purple)(1)) => (color(red)(3) xx color(purple)(1))/(color(blue)(4) xx color(green)(6)) => (cancel(color(red)(3)) xx color(purple)(1))/(color(blue)(4) xx cancel(color(green)(6))2) => 1/8#