Just put huge gobs of silicone and duct tape on there until you feel confident that it will hold. LOL
All seriousness aside, if you could get the top and bottom frame off without too much trouble, you could cut that silicone out of the side glass seams with a utility knife and remove the 2 broken pieces, get a new piece from glass shop, and carefully silicone it back in and replace the frame.
If you could find out if the side panel is tempered or not and stuff then you can determine what kind of glass to ask for.
But first thing I'd do is get the exact measurement & thickness, find out if tempered or not, and call a glass shop or two for a quote.
Another thing maybe maybe you ould do is contact the manufacturer of the aquarium and ask them to sell you a replacement piece. That might be even cheaper and easier if they would do that for you.... they probably won't do it though b/c they don't want peeps repairing their tanks. hehe
Just a few thoughts anyway.