Your exam score is low. I am sure you can do better / worse.

The sentence you provided is almost correct. However, it seems like you're trying to express two contrasting ideas. You might want to separate them into two different sentences or use a conjunction to connect them. Here are some suggestions: 1. "Your exam score is low. I am sure you can do better." 2. "Your exam score is low, but I am sure you can do better." 3. "Your exam score is low. If you don't study harder, you could do worse." Remember, "better" means to improve or exceed current performance while "worse" means to decline or perform less well than before.