How do I fix these strings forming....  

RE:

Short answer is you can't. FDM printing technology is not magic, it can't lay filament down in thin air. Those areas have no support from below so the filament being laid down there will inevitably sag. Of course there are things one can try. First of all, turn on supports. This will put material below those areas that you can then remove. But those surfaces will still look significantly worse than a bottom or top surface. Hard to tell how large the part is but you may get decent results just by bridging. I might model a sacrificial support into the part along the left open edge of the left area and then change the bridging angle so that the printer bridges across the shorter sides of those cutouts instead of the long sides, which is usually seems to do by default. 

But to get the best advice you should save the project as a 3mf file from prusaslicer, zip compress it (or the forum software won't accept it), and attach it here so we can see the model and your settings and suggest improvements. Fro what I'm seeing, it doesn't look like this part was designed with limitations of FDM printing in mind so maybe there isn't much that can be done. Sometimes it's also possible to cut the part into pieces that can all be printed without the need for supports and then assembled post printing. 

RE: How do I fix these strings forming....

Understand!  thank you for the reply - I will print it in parts as you suggest!