Find the determinants of the following matrices:
A=\left[\begin{array}{rr}1 & 2 \\-1 & 3\end{array}\right], \quad B=\left[\begin{array}{ll}4 & 1 \\2 & 1\end{array}\right], \quad \text { and } \quad C=\left[\begin{array}{ll}3 & 4 \\6 & 8\end{array}\right].