Answer
$2\sqrt{6}$
Work Step by Step
Substituting the given values of the variables, the expression $\sqrt{b^2-4ac}$ evaluates to
\begin{array}{l}\require{cancel}
\sqrt{(4)^2-4(2)(-1)}
\\\\=
\sqrt{16+8}
\\\\=
\sqrt{24}
\\\\=
\sqrt{4\cdot6}
\\\\=
\sqrt{(2)^2\cdot6}
\\\\=
2\sqrt{6}
.\end{array}