When I was at university, I did a course on Differential Equations where the 'Linear D-operator' was taught, i.e. that you introduce a a differential operator $D =\frac{d}{dx}$ so that for an example differential equation: 
\begin{equation}
\frac{d^2y}{dx^2} - a^2 y(x) = f(x)
\end{equation}
Becomes:
\begin{equation}
(D - a)(D + a) y(x) = f(x)
\end{equation}
And from there techniques are taught to resolve. I was wondering if anyone knew any good resources to learn this? are their any (preferably free) textbooks/tutorial videos/etc on this topic?