First, rewrite the expression as:
#(-4x)/1 -: x^2/10#
Now, use this rule for dividing fractions to simplify the 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)(-4x)/color(blue)(1))/(color(green)(x^2)/color(purple)(10)) => (color(red)(-4x) xx color(purple)(10))/(color(blue)(1) xx color(green)(x^2)) => (color(red)(-4color(black)(cancel(color(red)(x)))) xx color(purple)(10))/(color(blue)(1) xx cancel(color(green)(x^2))color(green)(x)) => -40/x#