You have to find the mean.
Add up all the points from the assignments and the 80.
Then Divide. For example, if there are 23 assignments in the grade book, divide by 23.
Now you have your final grade.
Example: 80+ 76 + 59 + 79 = 294
There are four assignments so you divide by four: 294÷4= 73.5
Final grade is 73.5
I hoped this helped.