Answer
$1605\;\rm Hz$
Work Step by Step
We have 5 antinodes and 4 nodes, as seen in the given figure, and as the author told us "both ends of the rod are anti-nodes".
We also know that this situation is similar to a sound wave in an open-open tube.
So its frequency is given by
$$f_m=\dfrac{mv}{2L}$$
And at the fundamental frequency, $m=1$
$$f_1=\dfrac{v_{\rm Al}}{2L}$$
Plugging the known;
$$f_1=\dfrac{6420}{2(4)}=\color{red}{\bf 1605}\;\rm Hz$$
