We can rewrite this expression as:
#3/(5x)root(3)(125a^6 * 3a^2b)#
We can now use this rule for radicals to simplify the radical:
#root(n)(color(red)(a) * color(blue)(b)) = root(n)(color(red)(a)) * root(n)(color(blue)(b))#
#3/(5x)root(3)(color(red)(125a^6) * color(blue)(3a^2b)) =>#
#3/(5x)root(3)(color(red)(125a^6))root(3)(color(blue)(3a^2b)) =>#
#(3/(5x) * 5a^2)root(3)(color(blue)(3a^2b)) =>#
#(3/(color(red)(cancel(color(black)(5)))x) * color(red)(cancel(color(black)(5)))a^2)root(3)(color(blue)(3a^2b)) =>#
#(3a^2)/xroot(3)(color(blue)(3a^2b))#
If necessary, we can go further:
#(3a^2root(3)(3a^2 * b))/x =>#
#(3a^2root(3)(3a^2)root(3)(b))/x =>#
#(3a^2(3a^2)^(1/3)root(3)(b))/x =>#
#((3a^2)^1(3a^2)^(1/3)root(3)(b))/x =>#
#((3a^2)^(1 + 1/3)root(3)(b))/x =>#
#((3a^2)^(3/3 + 1/3)root(3)(b))/x =>#
#((3a^2)^(4/3)root(3)(b))/x =>#
#((3a^2)^(4 * 1/3)root(3)(b))/x =>#
#(((3a^2)^4)^(1/3)root(3)(b))/x =>#
#((81a^8)^(1/3)root(3)(b))/x =>#
#(root(3)(81a^8)root(3)(b))/x =>#
#root(3)(81a^8 * b)/x =>#
#root(3)(81a^8b)/x =>#