Here,

$\begin{array}{l}
a=2 \\
b=-5 \sqrt{2} \\
c=4
\end{array}$

Discriminant $D$ is given by:

$\begin{array}{l}
D=b^{2}-4 a c \\
=(-5 \sqrt{2})^{2}-4 \times 2 \times 4 \\
=(25 \times 2)-32 \\
=50-32 \\
=18
\end{array}$