Equation For Needed Final Exam Grade

At my school, the semester is broken into 2 quarters that each get a grade. These two grades are then averaged to get an overall semester grade. Can someone provide me with an equation that can solve for the grade I need on the exam this quarter in order to get a certain grade for the semester, assuming I know what I got in the first quarter? In case that was confusing: Knowns:

1. What I want for the semester.
2. What I have so far this quarter.
3. The amount of this quarters grade the exam makes up.
4. What I got last quarter. Unknown:
5. What I need on the exam this quarter. Thanks.

Edit: I think I've figured it out. Is this right? (Overall Grade Wanted - (1st Quarter/2) - (1 - Percentage of Quarter Grade Exam Is Worth)(.5)(Current Grade))/((Percentage of Quarter Grade Exam Is Worth)*.5) = Exam Grade Needed