Hi Christian,

Maybe try to draw on a piece of paper or visual aid to determine the transformation of the rotation and reflection through. If you draw a cartesian coordinate plane with a point (eg. (1,1)) and try to see where it will appear after a rotation and then apply the reflection across your line representation you can combine those matrices to get the final transformation matrix. (maybe look up rotation matrix of 90 degrees first and adapt, the same for the diagonal line, through the origin from 3rd to 1st quadrant) 

The final form uses some vector a, rotation matrix R_a, translation matrix T_a, your solution should look sth. like this T_a * R_a (a) = a*