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(0) Draw the diagonal in the quadrilateral, opposite to the angle of 75°.
(1) Using the cosine law, calculate the length of this diagonal.
(2) Calculate the area "area_1" of the smaller triangle using the formula
area_1 = (half-product of two side lengths by sine of the angle between these sides).
(3) Calculate the area "area_2" of the larger triangle using the Heron's formula.
(4) Now the answer is this sum
area of the quadrilateral = area_1 + area_2.
Having these instructions, complete the solution on your own.