Answer :
[tex]x^2-x=72\\
x^2-x-72=0\\
x^2-9x+8x-72=0\\
x(x-9)+8(x-9)=0\\
(x+8)(x-9)=0\\
x=-8 \vee x=9[/tex]
[tex]x^2-x=72\\\\x^2-x-72=0\\\\a=1;\ b=-1;\ c=-72\\\\\Delta=b^2-4ac\to\Delta=(-1)^2-4(1)(-72)=1+288=289\\\\x=\frac{-b\pm\sqrt\Delta}{2a}\\\\\sqrt\Delta=\sqrt{289}=17\\\\x=\frac{-(-1)\pm17}{2(1)}=\frac{1\pm17}{2}\Rightarrow x=\frac{-16}{2}=-8\ or\ x=\frac{18}{2}=9[/tex]
