21. Given that the internal angles of hexagon $A B C D E F$ are all equal, the area of the triangle formed by the lines containing sides $A B, C D, E F$ is $192 \sqrt{3}$, and the area of the triangle formed by the lines containing sides $B C, D E, F A$ is $324 \sqrt{3}$. If the perimeter of hexagon $A B C D E F$ is $m+n \sqrt{p}\left(m, n, p \in \mathbf{Z}_{+}, \sqrt{p}\right.$ is a simplified radical), then the value of $m+n+p$ is ( ).
(A) 47
(B) 52
(C) 55
(D) 58
(E) 63
21. C.

Given that all the interior angles of the hexagon $A B C D E F$ are equal, we know that each interior angle is $120^{\circ}$. Therefore, the triangle formed by the lines $A B$, $C D$, and $E F$ is an equilateral triangle with an area of $192 \sqrt{3}$. Thus, the side length is $16 \sqrt{3}$, i.e.,
$$
A B+C D+E F=16 \sqrt{3} \text {. }
$$

Similarly, $B C+D E+F A=36$.
Hence, $m+n+p=36+16+3=55$.