`-22x^2+84x-72=0`

`<=>-22(x^2- 42/11 x + 36/11)=0`

`<=>x^2 - 2 . 21/11 . x + (21/11)^2 + 36/11 -(21/11)^2=0`

`<=>(x- 21/11)^2 =45/121`

`<=>`\(\left[ \begin{array}{l}x-\dfrac{21}{11}=\sqrt{\dfrac{45}{121}}\\x-\dfrac{21}{11}=-\sqrt{\dfrac{45}{121}}\end{array} \right.\) 

`<=>`\(\left[ \begin{array}{l}x=\dfrac{21}{11}+\sqrt{\dfrac{45}{121}}\\x=\dfrac{21}{11}-\sqrt{\dfrac{45}{121}}\end{array} \right.\) 

Vậy phương trình có tập nghiệm `S={21/11 + sqrt(45/121) ;21/11 - sqrt(45/121) }`