\(a,\Leftrightarrow x^2+6x+9+16-x^2=1\\ \Leftrightarrow6x=-24\Leftrightarrow x=-4\\ b,\Leftrightarrow8-12x+6x^2-x^3+27+x^3+6x-6x^2=17\\ \Leftrightarrow-6x=-18\Leftrightarrow x=3\\ c,\Leftrightarrow\left(x^2-1\right)^2=0\\ \Leftrightarrow\left(x-1\right)^2\left(x+1\right)^2=0\\ \Leftrightarrow\left[{}\begin{matrix}x=1\\x=-1\end{matrix}\right.\)