Determine the single matrix required for each of the following transformation combinations.

a. Rotation about the z-axis by 90° clockwise, followed by a stretch of factor 2 in the x-direction, followed by a reflection in the x-z plane.

b. Reflection in the plane x - y = 0, followed by an enlargement of factor 1/2 with centre of enlargement origin, followed by a rotation of 45° anticlockwise about the y-axis.