How do i clean my air filter on my 31cc 17" weed wacker? It starts, but dies about 6 seconds later.

You can clean the filter with standard soap and water insuring that it is dry when you re-assemble. Suggest lightly spraying it down with a light oil such as WD40 prior to the installation.

This unfortunately is not the common solution to your problem. Likely you either have:
1. A leak in a fuel line (often in the tank) or
2. A cracked primer bulb (if so equipped) or
3. A need for a replacement metering diaphragm within the carburetor. (They reduce there pliability and become ineffective),
4. An exhaust system that is partially plugged (some you simply can remove and clean and some it is very difficult).
4. A faulty fuel tank cap.

Above is the order I would attack the problem - good luck!