MY RH fender dettached from the car on its bottom left corner:

 

http://www.showcars-bodyparts.com/240z-p-fender-rh-newsteel.JPG

 

Should i hire someone to solder it back? I can see some rust underneath. It will be temporary until i finish replacing the cam and installing the ac system.

self tapping screw. They were held on with bolts so don't solder it.

If you can, weld on a heavy plate with the correct threaded hole in the same place as the original one.  That way you can protect it with rustoleum and copper antisieze, in case you need to remove the fender some time in the future.  You may also consider the cowl drain upgrade, so there is less stagnant water  on that area of potential rust.