I bought a brand new computer with Windows 10 operating system already pre-installed and had to transfer all of my games from my old computer. Took a while but I hoped that now I can play my games on ultra-high settings. The first game I tried to launch was ArcheAge and it failed to start and gave me d3dx9_42.dll is missing error. I have searched on the Internet and tried to perform various fixes, but nothing worked except this one.
This fix for d3dx9_42.dll is missing was posted by one guy on some forum and he said that it will fix d3dx9_42.dll is missing error in any application on any Windows operation system. All you have to do is to download and install d3dx9_42.dll from here: http://fix4dll.com/d3dx9_42_dll.
On the downloading page select the operating system if it was not detected automatically and follow the installation instructions. This fixed my issue on ArcheAge and other games like Civilization 5, Mafia 2 started without any error.