First, rewrite the expression as:
#(-5 * 81)/(63 * 40)((m^5 * m)/m^7)(n/n) =>#
#(-5 * 81)/(63 * 40)((m^5 * m)/m^7)1 =>#
#(-5 * 81)/(63 * 40)((m^5 * m)/m^7) =>#
#(-5 * 9 * 9)/(9 * 7 * 8 * 5)((m^5 * m)/m^7) =>#
#(-color(red)(cancel(color(black)(5))) * color(blue)(cancel(color(black)(9))) * 9)/(color(blue)(cancel(color(black)(9))) * 7 * 8 * color(red)(cancel(color(black)(5))))((m^5 * m)/m^7) =>#
#-9/56((m^5 * m)/m^7)#
Next, use these rules for exponents to simplify the #m# terms in the numerator:
#a = a^color(blue)(1)# and #x^color(red)(a) xx x^color(blue)(b) = x^(color(red)(a) + color(blue)(b))#
#-9/56((m^5 * m)/m^7) =>#
#-9/56((m^color(red)(5) * m^color(blue)(1))/m^7) =>#
#-9/56(m^(color(red)(5)+color(blue)(1))/m^7) =>#
#-9/56(m^6/m^7)#
Now, use these rules of exponents to complete the simplification:
#x^color(red)(a)/x^color(blue)(b) = 1/x^(color(blue)(b)-color(red)(a))# and #a^color(red)(1) = a#
#-9/56(m^color(red)(6)/m^color(blue)(7)) =>#
#-9/56(1/m^(color(blue)(7)-color(red)(6))) =>#
#-9/56(1/m^1) =>#
#-9/56(1/m) =>#
#-9/(56m)#
