SOLUTION
The mistake part is shown
The expression (-4)² was not properly simplified
This is done as follows
[tex]\begin{gathered} x=\frac{-4\pm\sqrt{(-4)^2-4(3)(20)}}{2(3)} \\ x=\frac{-4\pm\sqrt{16-240}}{6} \end{gathered}[/tex]
The next error is the addition
[tex]x=\frac{-4\pm\sqrt{-224}}{6}[/tex]
