Give me a moment, I'll see if I can dig up the info I was looking for. I think the DIY guy isn't going to be answering.
Edit: Okay, here's one link. I'll keep searching for more
Tips on fixing a crackd tank
Basically, it looks like you the the silicone out of the broken piece of glass, and pull it out (be careful with this). Run a bead of silicone around the edges of the open side of the tank and put the new piece of glass in place. Use duct tape to hold it together for at least 48 hours.
Edit the second:
building & repairing a fishtank
The second post in here is even better.