Assume we have two matrices, A and B, already defined:
 
 

Go to the home screen (by QUIT, if necessary), display matrices A and B:  ALPHA A ENTER ALPHA B ENTER
ALPHA A × ALPHA B ENTER

ALPHA A ENTER  3 × ALPHA A ENTER

We start with matrix B as shown above
Since we expect that "long decimal numbers" (like .333333333) are to be produced, we specify three decimal places of accuracy:  MODE, down arrow, right arrow (4), ENTER
Now, to get the inverse:  QUIT ALPHA B x-1 ENTER