Okay so, to get C you add the ones inside the triangle together.
42+58=100 Then the last angle will be 80, because when you add the three inside the triangle, you should get 180
42+58+80=180
To get D, you have to subtract 180 and 58, because 58+d will be a straight angle.
180-58=121
To get B and A, you would also have another straight angle (c, b, and a)
180-80=100
I believe that B=A, so, with the remaining 100, both A and B equal 50.
50+50+80=180
Does this help??