@mwiscount
In addition to the excellent information provided by @chriscas and @gnoack ,
It depends on how you are using the gradebook (points based or weighted) and how you want to do the extra credit. I think Chris's explanation is probably what you are looking for.
If you are using a points based gradebook, then your extra credit assignments will have to be worth 0 points and then you assign the points that the students earn (i.e. it looks like 5 out of 0). With points based you have to make the extra credit assignments worth 0 points or their point total is counted into the class total.
If you are using weighted groups and the extra credit work is to be in its own group, then what Chris has is what you want - thanks for that information Chris. I don't do things that way and have not had to think of how that would work.
The way I do extra credit is within each assignment group. in that case, I make the assignment worth 0 points and then assign the points total earned on the assignment for the grade. In this case, the total points earned is divided by the total points possible to get the score for that group. Below is a screenshot where the 4 assignments listed (1 is extra credit worth 0) are in the group weighted 100% for the course

Here the score is (8+8+31 + 3)/(10 + 12 + 31 + 0)*100% = 94.34% as indicated
Ron