Because the given conditions (7 centimeters, 9 centimeters, and 74°) are fixed and the unknown side length is fixed, the angles adjacent to the unknown side must also be fixed.
Given that a triangle has three sides and three angles
From the above question the unknown side of the triangle has a fixed value ( given ) hence the two angles adjacent to the unknown side will be <em>complementary angles</em>. this is because the sum of the angles of a triangle = 180°