The correct answer to (1 / 3A + 1 / 2b) (1 / 3a-1 / 2b)? Please explain the reason for each step and the whole process (the most important step is to apply the formula)

The correct answer to (1 / 3A + 1 / 2b) (1 / 3a-1 / 2b)? Please explain the reason for each step and the whole process (the most important step is to apply the formula)

=1/9a²-1/4b²