3 Answers

I hope it wasn't math.

That depends on the weight of your final exam on the total grade. If your final exam was worth 50% of the total grade, your final grade will be exactly half way between your score on the final exam and your grade prior to it.

Try your National exam board.

The consequence of that D- on your final could be pretty serious.  I think grades are usually comprised of homework scores, quiz scores, test scores, final score, and maybe participation, attendance, classwork.  Each of those scoring areas makes up a percentage of the grade.  If your combined averages for all the stuff was 91.31%, but the FINAL was HALF the grade, you would add 91.31 and 60 = 151.31 and divide by 2.  This would give you about a 76% overall average, most likely a "C".  
Find out how much weight the final carries.