Let \[f(x) =
\begin{cases}
x^2 + 1 &\text{if } x \leq 3, \\
bx + 4 &\text{if } x > 3.
\end{cases}
\]
Find the value of $b$ if the graph of $y = f(x)$ is continuous. Let's think step by step and output the final answer within \boxed{}.
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