Find the domain of $\frac{x^2 + 10x + 21}{x^2 + 4x - 21}$.  (Express your answer using interval notation.)
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Find the domain of $\frac{x^2 + 10x + 21}{x^2 + 4x - 21}$.  (Express your answer using interval notation.)
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
(-\infty, -7)\cup(-7, 3)\cup(3, \infty)
Find the domain of $\frac{x^2 + 10x + 21}{x^2 + 4x - 21}$.  (Express your answer using interval notation.)
train