Hey. I'm not sure. I went through it quick but this is a start:
First find where the light beam hits the surface. If we have a point which is the green point in the picture and where the light beam starts, and a vector direction , then a parametric representation of the line is . That line is the blue line in the figure. It's easy then to find where it hits the surface as we just substitute the parametric line into that equation and I get . Then it hits at the point . Now we need to calculate the normal at that point via . That's the red line. Then I'd calculate the angle between the blue and red which I calculated as which is almost 90 degrees.
Then the reflected line will have the same angle and follow the plane of the incident and normal (shown as the hatched surface). Then we'd need to parameterize the reflected line and then compute a t at which x=10 to find where it hits the plane x=10.