You can try:
SELECT employee.employee_number, employee.employee_name, division.division_name,
AVG(salary) AS AvgSalary
FROM employee
JOIN division ON division.division_id = employee.division_id
GROUP BY employee_number, employee_name, division_name
However, I'm not certain this will achieve your desired result.
To show the "average salary of all employees in each division", you'd do something such as:
SELECT division.division_name AS Division, AVG(salary) AS AvgSalary FROM division
JOIN employee ON employee.division_id = division.division_id
GROUP BY division_name
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