Find what grade you need on the final exam:
Current grade is 70% (or C-).
Final exam weight is 50%.
Required grade is 80% (or B-).
The final exam grade is equal to 100% times the required grade, minus 100% minus the final exam weight (w) times the current grade (g), divided by the final exam weight (w):
Final exam grade =
= ( required grade - (100% - w)×current grade ) / w
= ( 80% - (100% - 50%)×70% ) / 50% = 90%
So the final exam grade should be 90% (or A-).