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

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

MATRX select A ENTER  3 × MATRX select A ENTER 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 MATRX select B ENTER  x-1 ENTER
Use the right arrow to see the rest of the matrix