All the angles around a point on one side of a line add up to 180°.
A). Look at the angles at point-D, above the line.
You've got 40° and you've got a right angle ... 90°.
Those two angles add up to (40° + 90°) = 130°.
How much more is needed to make 180° ?
(180° - 130°) = 50°. That's angle 'x'.
B). Look at the angles at point-C, below the line.
You've got 55°.
How much more is needed to make 180° ?
(180° - 55°) = 125°. That's angle 'y'.