You can do this in three steps.

1) Draw a projection of your yellow line on the x-y plane, and also a line perpendicular to it (they are shown as blue lines). Also draw the green line in the image on the x-y plane.

2) Use the rotateaxis command to align your face as desired, as shown in the second picture. You should select the end points of the vertical green line as the axis to rotate about.

3) Use rotateaxis one more time, this time selecting the endpoints of the horizontal green line as the axis. Select the appropriate angle by picking the end of the vertical green line as the first reference, and the end of the yellow (orange in the picture) line as your second reference. The face normal is now aligned with the yellow line.

Dan

Thanks Michael and Dan for your prompt reactions!

Michael, your first method is a bit fuzzy to me, but don't bother explaining it otherwise, the second one is working for me, and I am glad I learned about the construction line trick!
Dan, your method is good, but also rotates the mirror along the Y axis, which is not desirable in this case.

but when reading all this, I suddenly realised I was focusing so much on the rotation aspect, that I forgot the orient tool, which beats the above solutions! simply select the plane and the normal line, select the orient tool and adjust base and target orientation. no additional construction lines to be made!

thanks, I wouldn't have found it without your solutions, and I learned a few things more about MoI!!!

rgds
wim

Hi wim, I'm glad you got it sorted now! And yeah I should have remembered the orient tool for that case myself - yes if you want to rotate a surface by realigning its surface normal to a different spot the orient tool is probably the easiest way.

- Michael