First, use this rule for exponents to rewrite the radical as:
#root(color(red)(n))(x) = x^(1/color(red)(n))#
#root(color(red)(3))(8a^51b^6) = (8a^51b^6)^(1/color(red)(3))#
Then rewrite the constant as:
#(2^3a^51b^6)^(1/color(red)(3))#
Now, use this rule of exponents to complete the simplification:
#(x^color(red)(a))^color(blue)(b) = x^(color(red)(a) xx color(blue)(b))#
#(2^color(red)(3)a^color(red)(51)b^color(red)(6))^color(blue)(1/3) =>#
#2^(color(red)(3) xx color(blue)(1/3))a^(color(red)(51) xx color(blue)(1/3))b^(color(red)(6) xx color(blue)(1/3)) =>#
#2^(color(red)(3)/color(blue)(3))a^(color(red)(51)/color(blue)(3))b^(color(red)(6)/color(blue)(3)) =>#
#2^1a^17b^2 =>#
#2a^17b^2#