Answer: 78°
Similar triangles are different from congruent triangles. Congruent triangles have equal angles and sides, while similar triangles have equal angles and proportional side lengths. In this case, because the triangles are similar, the angles should be equal. The sum of the angles of triangles are always 180°. Because triangle ABC has angle measures of 28° and 74°, the equation should appear as 28+74+x=180. Add the like terms to get 102. Subtract 102 on both sides to get x=78. Although we only solved for Triangle ABC, again, similar triangles have the same corresponding angle measures, so it applies to Triangle DEF, as well.