The expression $2z^2+13z+21$ can be written as $(z + a)(2z + b),$ where $a$ and $b$ are integers. What is $2a + b$?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The expression $2z^2+13z+21$ can be written as $(z + a)(2z + b),$ where $a$ and $b$ are integers. What is $2a + b$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
13
The expression $2z^2+13z+21$ can be written as $(z + a)(2z + b),$ where $a$ and $b$ are integers. What is $2a + b$?
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