Use the cosine law. Find smaller interior angle "a" 42^2 = 68^2 + 83^2 - 2*68*83*cos(a) cos(a) == 0.863660524. a = arccos(0.863660524) = 30.2698952 degrees. Hence, the largest interior angle of the parallelogram is 180 - 30.2698952 = 149.73 degrees (rounded). ANSWER