Trouble Opening a Fluorescent Light Fixture
It is bit difficult to tell from the picture.
Often on those older florescent fixtures the plastic panels have a integral plastic flange on their top edge that supports them- they basically hang inside the metal frame. So they drop in from the top- inside the fixture. To remove them lift up on the plastic and carefully flex and rotate them to get them out. If your fixture is real old then the plastic may have become more brittle and care should be taken not to damage them.