You're going to have to be a bit more specific, I'm afraid. You can't merely take your cum GPA from both places and find the average, even if they were both semester GPAs (In the case of the quarter GPAs it gets a bit more complicated).
To illustrate this, imagine I go to school for three years at a university and maintain a 3.9 GPA with taking 18 credits per semester, and finish up at another university taking 12 credits per semester and have a 2.5 GPA. Taking the average of 2.5 and 3.9 gives me 3.2 cum GPA; this, however, is not the correct cum because you didn't give proper weight to the amount of credits you accumulated with a 3.9. In fact, your GPA would be 3.46.
First, spend some time trying to calculate your semester gpa from your quarter gpa at the institution that gave you quarter grades. Then you should have no problem adding up everything appropriately. If you do, feel free to PM me and I can help out.