Is there a way to estimate your semester grade if you don't know your midterm grade? it's a extra credit question for math, and if i do it right it is added to my midterm grade.

No but there's a website where you can calculate what grade you need in order to get a certain grade in the class. Just add the extra credit grade to the grade you have first. 
Also, it's extra credit. Why wouldn't you do it? It's FREE points (sort of).

Just copy+paste this URL into your search bar:
http://finalexamcalculator.com/

METHOD 1=8.67

METHOD 2=11.83

Step-by-step explanation:

mean=average,just add and divide