When you know the length of the sides of a hexagon you use the next formula to find its area:
[tex]A=\frac{3\sqrt{3}}{2}s^2[/tex]s is the length of the sides
For the given hexagon:
[tex]\begin{gathered} A=\frac{3\sqrt{3}}{2}(20mm)\placeholder{⬚}^2 \\ \\ A=\frac{3\sqrt{3}}{2}(400mm^2) \\ \\ A=3\sqrt{3}(200mm^2) \\ \\ A=600\sqrt{3}mm^2 \\ \\ A\approx1039.2mm^2 \end{gathered}[/tex]Then, the area of the given hexagon is 1039.2 square millimeters