Answer :
[tex]7x^2=-21[/tex]
we need to clear x
[tex]\begin{gathered} x^2=\frac{-21}{7} \\ x^2=-3 \end{gathered}[/tex]we can't have a square root of a negative number in real numbers, if we use complex numbers we can obtain the next values for x
[tex]x=\pm i\sqrt[]{3}[/tex]