This question is just a fancy case of angle of incidence = angle of reflection. Instead of focusing on the blue lines, focus on the red arrow. By following these steps you will be able to draw your diagram:
1. Extend the red arrow until it touches the "mirror".
2. At the point where it touches the mirror, draw a normal (a line perpendicular from the surface of the mirror).
3. Mark the angle between the arrow and the normal as 45 degrees (this is what the question tells us).
4. Because angle of incidence = angle of reflection, you can draw a second arrow on the OTHER side of the normal, beginning where the first arrow ended, that is also 45 degrees away from the normal.
5. The new arrow points in the direction the waves will travel after being reflected. You can draw blue lines back in, perpendicular to that new arrow, to show how the waves are moving after being reflected.
Hopefully that made sense but I can clarify if need be.
