Answer
9
Work Step by Step
We find the determinant of the matrices using the rules of matrices. Doing this, we find:
$$ \det \begin{pmatrix}2&3&0\\ 1&4&-2\\ 2&-1&5\end{pmatrix}\\ 2\cdot \det \begin{pmatrix}4&-2\\ -1&5\end{pmatrix}-3\cdot \det \begin{pmatrix}1&-2\\ 2&5\end{pmatrix}+0\cdot \det \begin{pmatrix}1&4\\ 2&-1\end{pmatrix}\\\ 2\cdot \:18-3\cdot \:9+0\cdot \left(-9\right)\\ 9$$