see the attached figure with letters to better understand the problem
we know that in the triangle ABC ∡A+∡B+∡C=180° ∡B=2x-20 ∡C=180-(10x-151)----> 331-10x ∡A=180-(2x-20+331-10x)---> 180-(-8x+311)---> -131+8x applying the law of sines a/sin A=b/sin B a=b then sin A=sin B A=B----------> -131+8x=2x-20------> 8x-2x=131-20 6x=111 x=18.5°
