A Process in a Thread ?

I want to execute a process, but after the process is done i would like to execute a query so i know this process is done.
The process called ffmpeg is quite big so i start it and wait til its done with this coding:

ffmpeg.Start(); // start !                //ffmpeg.WaitForExit();                //ffmpeg.Close();  
ffmpeg.Start(); // start ! ffmpeg.WaitForExit(); ffmpeg.Close();




But it ta ...