Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What average week has shea stadium as the game site, and 1979-12-09 as the date?

### Inputs:
table_name: table, columns: ['Week', 'Date', 'Opponent', 'Result', 'Game site', 'Attendance']

### Response:
SELECT AVG Week FROM table WHERE Game site = shea stadium AND Date = 1979-12-09