Determine the magnitude and direction theata of F1 so that the resultant force is directed vertically upward and has a magnitude of 800N.

I have difficulty of where to start the problem? Do I sum up the forces of the x axis with unknowns in it and forces of the y axis with the unknown forces... where should I start... and how do I start solving the problem??

Please someone help me with this...

Thanks,

Nacha