first of all, the height of this room is quite... interesting (1,2 m
). Every room, especially the one not designed for studio use, needs measurements - it's kinda difficult to tell what to do without some tests, the setup of the speakers/soundsources etc. The hardest part to consider will be resonance made by three solid walls against the "weak" one. Absorbers should and would help - their placement is another story. I suggest to measure audio spectrum - that's the best way to start finding out the sweet spot for listening and potential problems.