[tex]\displaystyle\bf\\\frac{3a^{2} +6}{a^{3}+1 } -\frac{3}{a^{2} -a+1} -\frac{1}{a+1} =\\\\\\=\frac{3a^{2} +6-3\cdot(a+1)-1\cdot(a^{2} -a+1)}{(a+1)\cdot(a^{2} -a+1)} =\\\\\\=\frac{3a^{2} +6-3a-3-a^{2} +a-1}{(a+1)\cdot(a^{2} -a+1)} =\\\\\\=\frac{2a^{2} -2a+2}{(a+1)\cdot(a^{2} -a+1)} =\frac{2\cdot(a^{2} -a+1)}{(a+1)\cdot(a^{2} -a+1)} =\frac{2}{a+1}\\\\\\a=-1,4\\\\\frac{2}{a+1} =\frac{2}{-1,4+1}=-\frac{2}{0,4} =-\frac{20}{4}=-5\\\\\\Otvet \ : \ -5 }[/tex]