What is the sum of all numbers $a$ for which the graph of $y=x^2+a$ and the graph of $y=ax$ intersect one time?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the sum of all numbers $a$ for which the graph of $y=x^2+a$ and the graph of $y=ax$ intersect one time?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
4
What is the sum of all numbers $a$ for which the graph of $y=x^2+a$ and the graph of $y=ax$ intersect one time?
train