When solving problems like this, opposite angles will be equal to 180 when added together. So, when solving for C, you need to find x. You can do this by adding angles D and B.
3x + 9 + 2x - 4 = 180.
3x +2x + 9 - 4 = 180
5x + 5 = 180
5x + 5 - 5 = 180 - 5
5x = 175
5x/5 = 175/5
x = 35
Now, fill in x in angle A and C.
2x + 3 + C = 180
2(35) + 3 + C = 180
70 + 3 + C = 180
73 + C = 180
73 - 73 + C = 180 - 73
C = 107
The angle measure of angle C is 107. Hope this helps!