We plug in $x = 4$: \begin{align*}
3(4) + 2y &= 12\\
12 + 2y &= 12\\
y &= 0.
\end{align*}

Therefore, $b = \boxed{0}$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The point $(a, b)$ lies on the line with the equation $3x + 2y = 12.$ When $a = 4$, what is the value of $b$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
0
The point $(a, b)$ lies on the line with the equation $3x + 2y = 12.$ When $a = 4$, what is the value of $b$?
test