I can't run this code

Question

    \documentclass{article} 
    \usepackage{unicode-math}
    \setmathfont{XITS Math}

    \begin{document} 
    The quadratic formula is 
    \\\\
    $x = \dfrac{-b \pm \sqrt{b^2 - 4ac}}{2a}$
    \end{document}

I am running this program using Xelatex but it gives the error in line 3. !undefined control sequence and !Latex error Missing \begin{document}

Answer 1

\dfrac is defined in amsmath but never use \\ to force line breaks:

  \documentclass{article} 
    \usepackage{amsmath}
 \usepackage{unicode-math}
    \setmathfont{XITS Math}


    \begin{document} 
    The quadratic formula is 
    \[
    x = \dfrac{-b \pm \sqrt{b^2 - 4ac}}{2a}
     \]
    \end{document}