Find the domain of the function $a(x) = \sqrt{3^x - 7^x}.$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Find the domain of the function $a(x) = \sqrt{3^x - 7^x}.$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
(-\infty,0]
Find the domain of the function $a(x) = \sqrt{3^x - 7^x}.$
train