$a)$

$S+T=\left(17x^4-14x^2+7x-5\right)+\left(6x^3+11x^2-5x+5\right)=$

$=17x^4-\underline{14x^2}+\underline{\underline{7x}}-\underline{\underline{\underline{5}}}+6x^3+\underline{11x^2}-\underline{\underline{5x}}+\underline{\underline{\underline{5}}}=$

$=17x^4+6x^3-3x^2+2x$