-27
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If $\|\mathbf{a}\| = 3$ and $\|\mathbf{b}\| = 6,$ then find $(\mathbf{a} + \mathbf{b}) \cdot (\mathbf{a} - \mathbf{b}).$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
-27
If $\|\mathbf{a}\| = 3$ and $\|\mathbf{b}\| = 6,$ then find $(\mathbf{a} + \mathbf{b}) \cdot (\mathbf{a} - \mathbf{b}).$
train