Answer
$$-25$$
Work Step by Step
Using Cramer's rule to find the determinant of the matrix, we obtain:
$$ \det \begin{pmatrix}1&2&-1\\ 2&0&3\\ 0&1&5\end{pmatrix}\\ 1\cdot \det \begin{pmatrix}0&3\\ 1&5\end{pmatrix}-2\cdot \det \begin{pmatrix}2&3\\ 0&5\end{pmatrix}-1\cdot \det \begin{pmatrix}2&0\\ 0&1\end{pmatrix}\\ 1\cdot \left(-3\right)-2\cdot \:10-1\cdot \:2\\ -25$$
