For this, you need to know a concept in Trigonometry called "Law of Sines". This states that the ratio of any side of a triangle to the sine of the angle opposite is equal for all cases in that triangle.
Area of rectangle: 15 x 7 = 105 area of semicircle: 1/2 (3.14r^2) radius is half of diameter so it would be 7.5 1/2 (3.14(7.5)^2) =1/2 (176.71) =88.355 rounded is 88.4