I need help with the following problem.

Calculate the TOTAL actual cost for all over-budget projects managed by the manager named Graves.

I think I'm missing something from my answer, but I'm not sure how to get it.
I've written;
SELECT SUM (ACTUAL_COST)
FROM PROJECT, MANAGER
WHERE PROJECT.ACTUAL_COST = MANAGER.MG_NAME AND MG_NAME = GRAVES

Do I need to calculate the over-budget projects? If so, how do I write that?

Thanks in advance for your help!

First we would need to know the table structures. But your WHERE clause makes no sense. You are comparing apples and oranges.

Like ScottGem says, without your table structure, its difficult to say; however, the following may help you

SELECT SUM(Project.ActualCost)
FROM Project LEFT JOIN Manager ON Project.ManagerID = Manager.ManagerID
GROUP BY Manager.MG_Name
HAVING Manager.MG_Name = 'GRAVES'