Explanation:
area of quadrilateral = sum of area of triangles
area of triangle ADC = .5 * b * h
= .5 * 12 * 5 = 30 km^2
area of ∆ABC = .5 * sinx* ab* ac
= .5 * sin 30 * 3* 13 = 39/4 = 9.75 km^2
area of quadrilateral = 39.75 km^2
total cost =$ 397.5 million