Answer
a) $4$
b) $4$
Work Step by Step
a) Elements of the first column are $ 2, -1 $ and $ 5$. The first column of the array of signs is $+, -, +$.
$$\begin{vmatrix}
2 & 0 & 1\\
-1 & 3 & 2\\
5 & 1 & 4
\end{vmatrix}=2\begin{vmatrix}
3 & 2 \\
1 & 4
\end{vmatrix}-(-1)\begin{vmatrix}
0 & 1 \\
1 & 4
\end{vmatrix}+5\begin{vmatrix}
0 & 1 \\
3& 2
\end{vmatrix}$$
$$=2(12-2)+(0-1)+5(0-3)=20-1-15=4$$
b) Elements of the third row are $ 5, 1 $ and $ 4$. The third row of the array of signs is $+, -, +$.
$$\begin{vmatrix}
2 & 0 & 1\\
-1 & 3 & 2\\
5 & 1 & 4
\end{vmatrix}=5\begin{vmatrix}
0 & 1 \\
3 & 2
\end{vmatrix}-1\begin{vmatrix}
2 & 1 \\
-1 & 2
\end{vmatrix}+4\begin{vmatrix}
2 & 0 \\
-1& 3
\end{vmatrix}$$
$$=5(0-3)-1(4-(-1))+4(6-0)=-15-5+24=4$$