$a)\underline{2n^2}-\underline{\underline{3n^3}}-\underline{5n^2}+\underline{\underline{\underline{7n}}}-\underline{\underline{3n^3}}-\underline{\underline{\underline{9n}}}=$   $-6n^3-3n^2-2n$  

$b)\underline{-0,2x^2}-\underline{\underline{2x}}+\underline{1,9x^2}-\underline{\underline{x}}+\underline{0,2x^2}=$   $1,9x^2-3x$