We can divide the hexagon into 6 equilateral triangles, and call the side 'x'. I will draw the situation:
Every line is x because the hexagon is regular (all sides have the same length). Therefore we can find x, because two times x is 18'':
The hexagon has the same area of 6 equilateral triangles with the side equal to x. The formula of the area of an equilateral triangle is:
Therefore the area of the hexagon will be:
Rounding it will be 210 square inches.