d3dx9_43.dll
This file belongs to DirectX End-User Runtime. Install that package and the DLL arrives with it, in the right place and correctly registered.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
One of the legacy DirectX 9 helper libraries. Windows 10 and 11 include DirectX 12 but not these older per-version files, which games from the DirectX 9 era still expect to find.
Where you see it: Games from roughly 2005 to 2015. Easily the most common DirectX error there is.
This does not mean your DirectX is out of date. DirectX 12 comes with Windows and cannot be upgraded separately; the End-User Runtime simply adds the older helper files alongside it.
Install DirectX End-User Runtime
The official package, with the download URL Microsoft declares and the SHA-256 of every installer so you can verify it before running.
After downloading you can check the file against our index to confirm it is the installer the publisher shipped.