still having issues with matrix class

i hope i catch my favorite genius on here, who was helping me yesterday. i closed the thread before i actually tested the script because it made total sense. however, it's not working...

import flash.geom.Matrix;

var mat:Matrix = mc.transform.matrix; //where "mc" = the instance name
mat.b=Math.tan(45);

i wish to change the angle in which the y axis is skewed of a movie clip on the stage. any suggestions to what i'm doing wrong?