Do you want to find a matrix which represents these transformations on ?
In first case the reflection of point (x,y) in the line x = a will be
(x,y) -----> (( 2a - x ), y) and then in the x axis it will be ((2a-x) , -y )
Thus the reflection in both the lines ie., x=a and x axis taken together will be
(x,y) ----> ((2a-x), -y )
A reflection in y= x maps (x, y) to (y, x). Why don't you think about "reflection in the line y= -x"? If you are not sure draw a graph.And also
a reflection in y=x followed by a reflection in the line y=-x