Parallel lines
We know that if l and m are parallel there might be some angles that are equal to others:
Alternate interior angles
The angles closed by a blue circle are alternate interior angles, then they should be equal if l||m.
Vertcial angles
The angles closed by an orange circle are vertical angles, because they are opposite to each other on two crossed lines.
Now, we know two angles of the triangle of the middle:
We know that the addition of all the inner angles of a triangle is 180º. This is:
50º + 55º + xº = 180º
↓
105º + xº = 180º
Now, we can find x:
105º + xº = 180º
↓
xº = 180º - 105º
xº = 75º
Answer: xº = 75º
