So Angle GEN and FED are vertical angles, which means that their angles are congruent. Therefore, Angle FED is 60°
Angle GEN and DEN are a linear pair, which means that they are supplementary angles (add up to 180). To find Angle DEN, we have to form the equation: 60 + DEN = 180
For this, all you have to do is subtract 60 on both sides, and your answer will be: DEN = 120°