Verified answer

[tex]\displaystyle x^2+7-3x=0\\x^2-3x+7=0\\\\x=\frac{-(-3)б\sqrt{(-3)^2-4*1*7} }{2*1}=\frac{3б\sqrt{9-28} }2}=\frac{3б\sqrt{-19} }{2}=x \notin R[/tex]