Let $P=(a,b)$ be the point of intersection of the line $y=2x-10$ and the line through $(7,8)$ and $(9,0)$.  Compute $a+b$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Let $P=(a,b)$ be the point of intersection of the line $y=2x-10$ and the line through $(7,8)$ and $(9,0)$.  Compute $a+b$.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
13
Let $P=(a,b)$ be the point of intersection of the line $y=2x-10$ and the line through $(7,8)$ and $(9,0)$.  Compute $a+b$.
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