##12 root{3}(3)##
Work out the prime factorisations of the numbers under the square roots this will help simplify since roots split up;
##2 root{3}(81) + 3 root{3}(24) = 2root{3}(3^{4}} + 3 root{3}(2^{3} cdot 3) = 2root{3}(3 cdot 3^{3}} + 3 root{3}(2^{3}) cdot root{3}(3)##
##= 2 cdot root{3}(3) cdot root{3}(3^{3}) + 3 cdot 2 cdot root{3}(3)##
##=2 cdot 3 cdot root{3}(3) + 6 cdot root{3}(3) = 6 cdot root{3}(3) + 6 cdot root{3}(3) = 12 root{3}(3) ##