Equations reducible to the 2nd or 3rd degree;

Got it !

$\LARGE 6x^3-7x^2+7x+6=0$     Group 2 to 2!	$\LARGE S =\{-1;\frac{2}{3};3\} $
$\LARGE 2x^3+7x^2+7x+2 = 0$     Group 2 to 2!	$\LARGE S =\{-1;-\frac{1}{2};-2\} $
$\LARGE x^4-x^3-x+1= 0$	$\LARGE (x-1)(x^3-1)=0 \;\; ; \;\; (x-1)^2(x^2+x+1) =0 \;\; ; \;\; S =\{ 1 \}$
$\LARGE x^4-5x^3+5x-1= 0$	$\LARGE (x^2-1)(x^2+1)-5x(x^2-1)=0 \;\; ; \;\; S =\{1;-1;\frac{5+ \sqrt{21}}{2};\frac{5- \sqrt{21}}{2}\} $
$\LARGE x^3-31x^2+31x-5= 0$	$\LARGE S =\{1;\frac{1}{5};5\} $