$f(x) = (x + 1)^2$ for $x \ge 1$. $g(x)$ is a function whose graph is the reflection of the graph of $f(x)$ in the line $y = x$, then $g(x)$ is
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Always remember: "Reflection across $y = x$" is the code phrase for "Find the inverse function". The domain restriction ($x \ge 1$) is a critical hint that you must carefully consider whether to take the positive or negative branch when square rooting.