Flash MX 2004.
Math.invertMatrix(mat)
MatrixIndicates the Matrix object to invert (see Matrix object). It must have the following fields: a, b, c, d, tx, and ty.
A Matrix object that is the inverse of the original matrix.
MatrixMethod; Returns the inverse of the specified matrix.
The following example stores the currently selected object in the elt variable, assigns that matrix to the mat variable, and stores the inverse of the matrix in the inv variable:
var elt = fl.getDocumentDOM().selection[0];
var mat = elt.matrix;
var inv = fl.Math.invertMatrix(mat);