Explanation:
(a)
the sum of all internal angles in a polygon with n sides is
(n - 2)×180.
since in a regular polygon all angles are equally large, we get for one internal angle :
(n - 2)×180/n
for our hexagon this means the internal angle is
(6 - 2)×180/6 = 4×30 = 120°
(b)
the area of the hexagon is (as the graphic so nicely suggests) the sum of the areas of all 6 inner triangles.
the area of a triangle is
baseline × height / 2
baseline = 10 cm
the height (from the tip of the triangle down to the baseline in a right angle) is half of 17.3 cm, because these 17.3 cm are the sum of the 2 heights of 2 triangles placed tip to tip.
so, height = 17.3/2 = 8.65 cm
the area of one of the triangles is therefore
10 × 8.65 / 2 = 5 × 8.65 = 43.25 cm²
the total area of the hexagon is then
6 × 43.25 = 259.5 cm²