If f is parallel to g, then c should equal 75 degrees because c and 75 degrees are vertical angles. If C is 75 degrees then D must equal 105 degrees because c and d are both on the left side of line f which means they must add up to 180 degrees, which should get you to this equation: c + d = 180. We can replace c for 75 because we already known that, which should get you: 75 + d = 180. So you should then subtract 75 against 180 to get your answer: d = 105.
