Evaluate the following determinants and negative of determinants of order two:
(a) $\left|\begin{array}{cc}2 & 5 \\ 3 & 6\end{array}\right|, \quad\left|\begin{array}{cc}3 & -6 \\ 1 & -4\end{array}\right|, \quad\left|\begin{array}{rr}-4 & -2 \\ 7 & -3\end{array}\right|$
(b) $-\left|\begin{array}{cc}6 & 4 \\ 7 & 5\end{array}\right|, \quad-\left|\begin{array}{rr}1 & -3 \\ 2 & 4\end{array}\right|, \quad-\left|\begin{array}{rr}8 & -3 \\ -6 & -2\end{array}\right|$