x^2-7x=-9
x^2-2*x*(7/2)+(7/2)^2=-9+(7/2)^2
(x-7/2)^2=13/4=(±√13/2)^2
x-7/2=±√13/2
x=7/2±√13/2
所以x1=(7+√13)/2,x2=(7-√13)/2