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°
the answer is
x=18.5 degrees