Help needed to understand the operator [1/(D-a)] in derivation

[rajdeep.ee]

In the derivation to find the Particular Intergral of a linear Diff. Eq. with constant coefficients where Q (the right hand side of the equation which is a function of x only) is of the form e^(ax) where 'a' is any constant, I come across the operation [1/(D-a)] e^ax at a step which when evaluated in the next step yields {e^ax (integration of) e^ax e^-ax dx}. Here D stands for differentiation, so that 1/D stand for Integration. I wanted help in understanding the operator [1/(D-a)] as I am unable to interpret its operation on e^ax.

Thank you.