The whole triangle has to equal 180°, so if one angle is 40° then you need to figure out what the other angles are.
180 - 40 = 140
140 / 2 = 70
So now you found the value of both angles BUT we are trying to solve for h.
180 - 70 = 110
110 / 2 = 55
55 = h
I hope this helps love! :)
