Hi!
X=20
First of all,all triangles add up to 180. You were given two points,40 & 40. If you add those together,you get 80. 180-80=100. That being said all lines also equal to 180. If D=100, then 180-100=80,so that angle on the outside would also equal 80. So you would do the following:
3x+20=80
From there you would subtract 20 from 20 and 80 and you would be left with
3x=60
From there you would divide 3 from each side leaving you with
X=20
Hope this helps!