The angle you are trying to find is an alternate interior angle with 3x + 15.
First find x. Make 3x + 15 and 10x + 9 add up to 180.
3x + 15 + 10x + 9 = 180
13x + 24 = 180
13x = 156
x = 12
Now plug in 12 for x in 3x + 15.
3(12) + 15
36 + 15
51
The angle 3x + 15 = 51. Alternate interior angles are equal to each other, so the angle you are trying to find is also equal to 51°.