Answer: You know when an expression is in it's simplest form when you can't do anything to it.
Explanation:
Example: -3x + 100 > -50
You can break this equation down into tinier pieces and make it easier to understand.
-3x > -150
This is a simpler inequality than the first one but, not in simplest form.
Now, x<50 is -3x+100>-50 in it's simplest form. You can't do anything to x>50 anymore. It's in simplest form.