Let $M = \left[ \begin{array} { r r } 1 & 1 \\ - 2 & 4 \end{array} \right]$ and $X = \left[ \begin{array} { l } 1 \\ 2 \end{array} \right]$ such that
$$\begin{aligned}
& \mathrm { M } \cdot \mathrm { X } = \mathrm { aX } \\
& \mathrm { M } ^ { - 1 } \cdot \mathrm { X } = \mathrm { bX }
\end{aligned}$$
For real numbers a and b satisfying these equalities, what is the sum $a + b$?\\
A) $\frac { 1 } { 3 }$\\
B) $\frac { 4 } { 3 }$\\
C) $\frac { 5 } { 3 }$\\
D) $\frac { 8 } { 3 }$\\
E) $\frac { 10 } { 3 }$