Answer
$$27+6\sqrt{14}$$
Work Step by Step
We are told:
$$f(x)=x^2$$
Thus, plugging in the given value for $x$, we find:
$$ \left(\sqrt{6}+\sqrt{21}\right)^2\\ \left(\sqrt{6}\right)^2+2\sqrt{6}\sqrt{21}+\left(\sqrt{21}\right)^2\\ 27+6\sqrt{14}$$
