Find the Determinant of the Matrix $\left[ \begin{array} { c c } 7 & 1 \\- 1 & 2\end{array} \right]$

Find the determinant of the matrix. $\left[ \begin{array} { c c } 7 & 1 \\- 1 & 2\end{array} \right]$

A) 15
B) 13
C) $- 13$
D) $- \frac { 1 } { 13 }$
E) $- 15$