Now look at the straight line below,We know that in a straight line sum of all angles is equal to 180°. so the remaining unknown angle in that line is 180°-150°=30° and that angle is equal to the angle in the upper straight line as they are vertically opposite angles in the intersection of two straight lines.
Now apply the same concept for the upper straight line also that is sum of the angles 30° and 50° and 2x° is equal to 180° .