First, you know that c and d add up to the same degree so you can solve that bottom triangle. A triangle adds up to 180 degrees and 40+40-180 is 100 which is our missing angle. The two other connecting sides are also equivalent to this new angle(100) which helps you solve the triangle to the right. You take the given angle of A(40) which is equivalent to b and add 100. After you would subtract the sum of the triangle which is 40. For your final answer you would take this new angle and add it to angle d to get your answer 80.