Form a differential equation representing the given family of curves by eliminating arbitrary constants a and b. $y=a\: e^{3x}+b\: e^{-2x}$

Question

$\begin{array}{1 1} (A)\;y'' - y' - 6y = 0. \\(B)\;y'' - y' - y = 0. \\ (C)\;y'' - y - 6y' = 0. \\ (D)\;y''' - y' - 6y = 0. \end{array}$