How to mirror a bitmap and then rotate

Question

I have a bitmap that I mirror using a matrix

   mMirror.postScale(-1.0f, 1.0f);

and can "not mirror" use

   mMirror.postScale(1.0f, 1.0f);

However, now when I rotate the bitmap with mRRight.postRotate(90), the mirroring is lost. Why?

EDIT Highlighted part of the answer for. When you create a Bitmap object, the Identity matrix is ​​used and created internally, which corrects all mirroring, etc.

Now the question will change on how to mirror and then rotate the matrix.