a
(a) $\left| {\,\begin{array}{*{20}{c}}x&3&7\\2&x&2\\7&6&x\end{array}\,} \right|\, = 0$
$ \Rightarrow $ $(x + 9)\,\left| {\,\begin{array}{*{20}{c}}1&1&1\\2&x&2\\7&6&x\end{array}\,} \right| = 0$,
by ${R_1} \to {R_1} + {R_2} + {R_3}$
$ \Rightarrow $ $(x + 9)\,\{ ({x^2} - 12) - (2x - 14) + (12 - 7x)\} = 0$
$ \Rightarrow $ $(x + 9)\,({x^2} - 9x + 14) = 0$
$ \Rightarrow (x + 9)(x - 2)\,(x - 7) = 0$
Hence the other two roots are $x = 2,\,7$.