Hey there! :D
The congruence marks on the trapezoid make the angles congruent. This makes opposite angles supplementary. (Because all the angles in a trapezoid add up to 360 degrees)
12x-31+6x-5= 180
18x-36= 180
Add 36 to both sides.
18x= 216
Divide both sides by 18.
x= 12
You can just plug that in to find m<H.
6x-5
6(12)-5
72-5
m<H= 67
I hope this helps!
~kaikers