We are given that two lines are cutting by a transversal line.
We can see that angle <x and angle <y are Consecutive Exterior Angles.
Note: When two parallel lines cut by a transversal line, the each pair of angles outside the parallel lines on the same side of the transversal are called Consecutive Exterior Angles.
Also please note that two Consecutive Exterior Angles are supplementary.
In simple words, the sum of two Consecutive Exterior Angles is 180 degree.
So, we can apply supplementary angles property of Consecutive Exterior Angles to find the sum of x and y in the given diagram.
Therefore, x+y = 180°.
