[tex]\Large\boxed{\begin{array}{l}\tt a)~\sf x^2-6x+5=0\\\sf\Delta=b^2-4ac\\\sf\Delta=(-6)^2-4\cdot1\cdot5\\\sf\Delta=36-20\\\sf\Delta=16\\\sf x=\dfrac{-b\pm\sqrt{\Delta}}{2a}\\\\\sf x=\dfrac{-(-6)\pm\sqrt{16}}{2\cdot1}\\\\\sf x=\dfrac{6\pm4}{2}\begin{cases}\sf x_1=\dfrac{6+4}{2}=\dfrac{10}{2}=5\\\\\sf x_2=\dfrac{6-4}{2}=\dfrac{2}{2}=1\end{cases}\\\sf S=\{1,5\}\end{array}}[/tex]
[tex]\Large\boxed{\begin{array}{l}\tt b)~\sf 3x^2+4x+1=0\\\sf\Delta=b^2-4ac\\\sf\Delta=4^2-4\cdot3\cdot1\\\sf\Delta=16-12\\\sf\Delta=4\\\sf x=\dfrac{-b\pm\sqrt{\Delta}}{2a}\\\\\sf x=\dfrac{-4\pm\sqrt{4}}{2\cdot3}\\\\\sf x=\dfrac{-4\pm2}{6}\begin{cases}\sf x_1=\dfrac{-4+2}{6}=-\dfrac{2}{6}=-\dfrac{1}{3}\\\\\sf x_2=\dfrac{-4-2}{6}=-\dfrac{6}{6}=-1\end{cases}\\\\\sf S=\bigg\{-\dfrac{1}{3},-1\bigg\}\end{array}}[/tex]
[tex]\Large\boxed{\begin{array}{l}\tt c)~\sf x^2+3x-28=0\\\sf\Delta=b^2-4ac\\\sf\Delta=3^2-4\cdot1\cdot(-28)\\\sf\Delta=9+112\\\sf\Delta=121\\\sf x=\dfrac{-b\pm\sqrt{\Delta}}{2a}\\\\\sf x=\dfrac{-3\pm\sqrt{121}}{2\cdot1}\\\\\sf x=\dfrac{-3\pm11}{2}\begin{cases}\sf x_1=\dfrac{-3+11}{2}=\dfrac{8}{2}=4\\\\\sf x_2=\dfrac{-3-11}{2}=-\dfrac{14}{2}=-7\end{cases}\\\sf S=\{-7,4\}\end{array}}[/tex]
[tex]\Large\boxed{\begin{array}{l}\tt d)~\sf -x^2+9x-20=0\cdot(-1)\\\sf x^2-9x+20=0\\\sf\Delta=b^2-4ac\\\sf\Delta=(-9)^2-4\cdot1\cdot20\\\sf\Delta=81-80\\\sf\Delta=1\\\sf x=\dfrac{-b\pm\sqrt{\Delta}}{2a}\\\\\sf x=\dfrac{-(-9)\pm\sqrt{1}}{2\cdot1}\\\\\sf x=\dfrac{9\pm1}{2}\begin{cases}\sf x_1=\dfrac{9+1}{2}=\dfrac{10}{2}=5\\\\\sf x_2=\dfrac{9-1}{2}=\dfrac{8}{2}=4\end{cases}\\\sf S=\{4,5\}\end{array}}[/tex][tex]\Large\boxed{\begin{array}{l}\tt e)~\sf 3x^2-4x+2=0\\\sf\Delta=b^2-4ac\\\sf\Delta=(-4)^2-4\cdot3\cdot2\\\sf\Delta=16-24\\\sf\Delta=-8<0\\\sf N\tilde ao~existem~ra\acute izes~reais.\end{array}}[/tex]