If the course names are in column A, the credit-hours in column B and the
letter grades in column C, try:
=SUMPRODUCT((B1:B5)*((69-CODE(LEFT(C1:C5,1))+(LEFT(C1:C5,1)="F"))+((RIGHT(C1
:C5,1)="+")-(RIGHT(C1:C5,1)="-"))/3))/SUM(B1:B5)
assuming the grades are A, B, C, D and F with pluses and minuses.
