what do we need to know:
the radius of the semi circles;
the area of:
purple region = area of a circle = pi * r^2;
green region = base * height;
what we know:
the base, 28m;
what the sum of the two diameters is, 28;
the length of a single diameter, 14;
what we can assume:
the two semi-circles are equal, as otherwise i cannot see a way to answer this;
the radius of the two semi-circles, 7m as the two diameters = 28, & d/2 = r
what does that mean:
the height is 17m
green region area = 476 m^2
purple region area is = 153.96 m^2