d3dx9_39.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.
The error names your specific file, but the fix is the same for the whole family: the whole DirectX family and the one runtime that installs all of it.
If the file is already there and Windows still complains, read the message again: “entry point not found” is a different problem from a missing file, and installing the runtime again will not solve it.
And if the box says RUNDLL rather than naming a program, the fix runs the other way: something left an instruction behind pointing at a file that is meant to be gone, so restoring the file is the wrong move.
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.
The same goes for the tools that offer to do it for you: what a DLL repair tool can and cannot actually fix.
What it is
An earlier build of the same DirectX 9 helper library. Each game was compiled against one specific version, which is why the numbers differ between titles.
Where you see it: Games from the late 2000s.
The End-User Runtime installs every version at once, so you do not need to match the number yourself.
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.